We know we've played something great, perhaps even something special, when we find ourselves thinking about it when we're not playing it. When we find ourselves wishing we were playing it while we're sat on the underground, or browsing the internet, or listening to our editor prattle on about Geometry Wars 2. It doesn't happen very often, but when it does it reminds us of the power video games hold over us, how entrancing the spell they cast really can be. It happened again recently, and the game was Fallout 3.

It seems ridiculous to have to form some kind of informed opinion based on a two hour toe-dip into Bethesda's stunning post-apocalyptic world, given the gargantuan nature of this sci-fi RPG, but that's what we're paid to do, so here goes. Haters be quiet - Fallout 3 is shaping up to one of the best games of 2008, and, fingers crossed, could be one of the best RPGs ever.

Amoral, subversive and pulp. It sounds like we're describing a particularly bad kind of orange juice, but it's actually the three words our new previews editor Neon Kelly came up with when we asked him to sum up the Fallout series, a series that a lot of PC gamers still care a great deal about.

You can't, of course, please everyone, and Bethesda knows this. It also knows that, actually, if it can make a game as good as The Elder Scrolls IV: Oblivion, it'll have done it's job. From our time with the game, Fallout 3 might even be better than Oblivion.

You may be wondering why we're comparing Fallout 3 to Oblivion in the first place. It's because the game could almost be Oblivion 2, or, as some journalists are calling it, Oblivion with guns. What's certain is that it feels very similar. The trademark vistas, the dialogue system, the camera angle when you talk to NPCs, the instant teleportation to already discovered locations, it's all there.

Simply saying Fallout 3 is Oblivion with guns doesn't do Bethesda's hard work justice. Fallout 3 looks better than Oblivion, even though it's not finished. Fact. The draw distance is mind-blowing - when we leave Vault 101 for the first time, the giant doorway creaking open, our character's eyes adjusting to the first natural light he's ever been exposed to, the nuke-ravaged Washington DC that lies before us almost knocks us out. Dusty settlements, what looks like a small town, hills, whistling wind, it's all there. We believe. We believe that this actually is what a US city would look like if a nuclear war landed on top of it. We can see the Washington Monument (less important for us Brits) in the distance, somehow still standing. "You can travel there and go all the way up to the top," Pete Hines, Bethesda's communications chief tells us. We believe him.

Usually when we're granted precious hands-on time with games, we're given a very linear, carefully crafted snap shot of what it's got to offer. Not so here. Bar a couple of restrictions centred around the main 'find your father' plot (which Bethesda won't be talking about until the game is released), Pete tells us we have the freedom to go anywhere, do anything, just the way open world games should be played. We take him up on his offer.

It's the little things that have us so excited. Straight away we head left, off the beaten path that leads to the town of Megaton, the first main quest hub players will experience after the 40 minutes or so they've spent in Vault 101. We spot a sign that says 'Scenic Overlook'. We stand still, using the thumb stick to pan the camera, taking in the desperate, charred landscape. The sign is the first piece of evidence, if it was needed, that Bethesda is well aware of the series' trademark humour.

We level up - a specially constructed ding designed to give us a taste of how the RPG-ness of Fallout 3 works. The Pip-Boy appears on screen - a wrist-mounted computer that monitors your health, allows you to assign skill points, lets you view the world map and track quests, to name but a few of its functions. We have 13 options, all staple RPG categories - barter, repair, lockpick, sneak etc - in which to sink our attribute points. Some will be familiar to Fallout fans. Big guns, for example, improves your effectiveness with oversized weapons like the Fat Man (really), flamethrowers, missile launchers and the like. We're guessing you can work out for yourself what the Small Guns skill improves. But it looks like Bethesda has ditched a couple of Fallout skills - outdoorsman, the random encounter controller that wouldn't have worked anyway, and gambling, which was rarely used.

The game begins inside the vault but soon becomes very vast indeed

The character customisation doesn't end there. Whenever you level up you're given a point to spend on perks, special abilities that help you fine tune your character even further. Daddy's Boy, for example, grants +5 to your science and medical skills. Intense Training allows you to put a single point into any of your S.P.E.C.I.A.L. attributes. Lady Killer gives you +10% damage against female opponents. There are loads more, some we know, but can't talk about (sorry, you know how these things work).

We go travelling. We know that we're supposed to go to Megaton, our only lead on our father, and we know the way we're supposed to go, via arrows on a slider on the bottom left hand side of the screen, but we decide not to. Not yet, anyway. We discover what looks like the deserted Springvale settlement. We pass an Enclave Eyebot - a floating mechanical eye that spews propaganda from President Eden (voiced by A Clockwork Orange star Malcolm McDowell), a creepy, almost cult-like figure who runs the Enclave, an organisation that fancies itself the new US government.

We come across a merchant, called Crow, and get a taste for Fallout 3's dialogue system, which proves to be almost identical to Oblivion's. We run about, a lot, trying to find something, anything to kill. We struggle - our first concern. Yes, this is a post-apocalyptic wasteland. Yes, it's not going to be as busy as present day Washington DC, but the last thing we want to feel is bored.

We preferred the VATs targeting system

We arrive at the gates of Megaton, a shanty-town-esque collection of buildings cobbled together around an unexploded bomb. We're greeted by Lucas Smith, the town sheriff. We have three dialogue options, one obviously good, one obviously bad and one neutral. We play nice, offering to disarm the bomb that sits buried in the ground at the centre of the settlement. Our speech skill isn't high enough to get more caps, Fallout 3's in-game currency, as reward, but the quest is very much on, and, just to make sure we're aware of that fact, a strong, capitalised white font slowly appears on the left hand side of the screen - THE POWER OF THE ATOM - along with a super cool sound effect. He also points us in the direction of the local inn, and suggests someone there might know where our father is. Nice bloke.

And then, we kind of flip out, and decide that, actually, we fancy shaking things up a bit. We shoot a Megaton trader's cow with our weak pistol, which obviously doesn't go down well with the trader, or the general Megaton populace. They turn on me, shooting, punching and kicking. There is no escape. We're dead, killed over a cow.

We start again, this time picking the not so nice dialogue options with the sheriff. Interestingly, this time he doesn't point us in the direction of the inn. We haven't exactly put Fallout 3's karma system through its paces, but we can tell that at least the way you'll go about unravelling the story will change depending on how you role play. We know that you can decide to blow Megaton up, or to save it but how much it changes it, a crucial question in our eyes, remains to be seen.

Down at the protruding bomb, an old man is knelt in worship. He's part of a religion that worships the bomb, of all things, and comes across as a bit barmy. But, aware that we're on a time limit, and the fact that we haven't explored the combat system, we leave Megaton and go in search of stuff to kill. A few minutes later and we come across a Bloatfly - a hideous-looking bird bat thing that's clearly got it in for us.

Time to use VATS - Vault-Tec Assisted Targeting. With a single button press we freeze time and enter a new mode. The camera focuses on our enemy, and separates out the various body parts Robocop style. Each has a clearly displayed chance to hit. Then, using Action Points, displayed on the bottom right corner of the screen, we can cue up shots both on different body parts and, using the thumb stick to shift, different enemies within range.

Confirm your selection and the camera will switch into third person and super slow motion, showing bullets, laser fire, even missile fire slowly hit your enemy. Just like in the first two Fallout games, there's a tactical element to VATS - cripple the arm of an enemy and it can't fire its weapon. Shoot the legs off and it can't move. But the gore some hits can result in is something else. Decapitations, dismemberment, groans, moans, flying bodies, the spray of blood on the camera - it's all there, in super crisp high definition. It's truly shocking, and we can't help but laugh.

It's incredibly satisfying, and, crucially, fun. Pausing the action and cueing up attacks is classic Fallout, and is sure to please RPG fans the world over. It's miles better than simply playing the game like a straight-up action game, which doesn't feel as tight as other shooters. But because of the way the Action Point system works, in that they slowly replenish over time, you won't be able to constantly use VATS. We found that we used it, then had to use the normal real-time FPS mode while we impatiently waited for our Action Points to increase. In a way, VATS feels so good it shows the normal shooting up, and we suspect we'll end up wishing for more Action Points than we'll get.

Small triangles on the slider radius point us towards areas of interest, which is particularly useful while exploring. We uncover Springvale Elementary School, deserted, barren. We tune into Enclave radio and listen to Malcolm McDowell talk about baseball, a game people used to play before the nuclear war. We see retro 50s style posters - one advertising the Captain Cosmos Show, another for Sugar Bombs cereal. It's a world that's teeming with life, without actually having much life out in the wild.

Don't get us wrong, enemies aren't sparse, if you go in search of them. We're given a quest from a Megaton shopkeeper to find medicine from the nearby Super Duper Mart, a giant K-Mart inspired building with abandoned trolleys littered outside. Inside we spark a shootout with a group of demented raiders, using VATS to pop heads, tear off limbs and cripple torsos. We try the lock picking mini-game, which requires you to use the thumb sticks like independent lock picks to open cases and boxes. We find the medicine hidden on a shelf, and trigger an instruction to bring it back to the NPC. So very RPG, so very Oblivion.

Oblivion fans will feel somewhat at home

We also smell elements of 2K Games' tremendous FPS BioShock seeping through Fallout 3's virtual pores. Yes it's open world, but it's an open world devastated, a world filled with horror and a world with a million stories to tell. It's a world that tells us what's happened without slapping us in the face with the information. And we like that.

But it's also a world with a few technical problems. We noticed a degree of texture pop up as we explored the world. The camera sometimes has a fit when in VATS slow motion mode. The third-person perspective we imagine will go largely unused. And at one point we were forced to reboot the game after we got stuck under part of a collapsed bridge - fast travel wouldn't work because the game thought we were falling. Our hope, and our belief, is that Fallout 3 won't be let down by any technical issues that might make it into the released code.

So, as we head back to the office, our hands-on time over, we find ourselves whistling the Marching Band tune, without even realising it. This sums up how we feel about Fallout 3. We've hardly scratched the surface of what the game has to offer and we're desperate to play it again. We want to eek out every nook and cranny, walk down every alley, talk to every NPC and absorb ourselves completely in what could prove to be Bethesda's best game to date. This is undoubtedly a massive world, with incredible attention to detail, and a story behind every corner. The best thing? Bethesda promises even more surprises as the game nears its projected fall 2008 release. We can't wait.

Fallout 3 is due out for Xbox 360, PS3 and PC late 2008.