I'm trying to imagine what a shorter, slightly chubbier, eight year-old version of myself would think if he were to lay his eyes on Sonic Generations. What would that cheeky little scamp think of 2011's lanky Sonic zipping round Green Hill or Chemical Plant Zone in three dimensions? He'd stare, most likely, wide-eyed, mouth open, bewildered that the levels he knows and loves could look so different - and yet so familiar.

It's similar to how the me of today regards iconic 3D environments - Sonic Adventure's Speed Highway for example - which have been re-imagined in 2D. Generations is very much a game of two halves; it's about classic levels brought to life in 3D, designed with modern Sonic's skill-set in mind, alongside modern levels with their third dimension shaved off, letting the little blue hedgehog of yesteryear have a pop. Old and new, 2D and 3D - a game that panders to the you of today and the you of the early nineties in equal quantities.

Really, though, it's an excuse to celebrate 20 years of Sonic; a reason to mess with the formula of well-known levels and put a new spin dash on things. It's fan service, ultimately, and SEGA is pulling out all the stops.

A hub world connects the zones, each split into two acts: a classic 2D stage and a modern 3D one, and pressing the Triangle button (or Y, on Xbox) from the hub switches between the two hedgehogs. The first zone, Green Hill, is just as you'd expect to find it in its 2D form: green, with sunflowers, bridges (with jumping jaws badniks) and plenty of loop-de-loops. The depth of field is more impressive than it was back in the day, though, and Sonic isn't afraid to flit between foreground and background should the need arise. It looks great, with impressive animations and familiar physics. This is how Sonic 4 should have looked and felt.

In 3D, the level retains the same aesthetic, except enemies are dotted about in just such a way that modern Sonic can home from one to the next. It's not limited to just 3D, either, and every now and again things will return to a 2D plane - much like it did in Colours. There's also the obligatory grinding sections, where Sonic leaps from rail to rail, taking the correct path as not to fall to his death. Each level comes with its own set piece, too. In terms of Green Hill Zone, this means a giant Jaws badnik chasing Sonic through watery caverns beneath the hill.

A common complaint of the 3D Sonic offerings is that you can simply hold forward, whack the jump button every once in a while, and you'll probably reach the end in one piece. Good luck doing that this time around. The level design has seen noticeable improvements, making the 3D side of the game a more involving affair than before. For zones like Chemical Plant, which have never been given the three dimensional treatment before, it's particularly impressive. Seeing level gimmicks, such as the globules of oil that leap from holes in the ground, introduced to 3D areas will come as a particular treat for fans.

Those familiar with the series will also love the soundtrack, which features remixes of original themes (again, Chemcial Plant Zone was a highlight for me), and mash-ups of several zones in one for an all new - yet very familiar - musical offering. You can even unlock new tracks, and then choose which you want to listen to in certain levels.

Other levels I got to try my hand at include Sonic and Knuckles' Sky Sanctuary, Sonic Adventure's Speed Highway and Adventure 2's City Escape, which kicks off with Sonic snowboarding down the hills of a busy urban metropolis. The hub is a 2D level in itself, too, with five 'Challenge' missions which can be found after a bit of jumping about. Here, SEGA continues to play around with the tropes of each zone, introducing new ways to play and adding new mechanics via other characters.

You don't actually play as, say, Knuckles, but in one mission you can take advantage of the red echidna's fists, where a tap of the button sends him burrowing into the ground for rings. As well as this there are time attacks, high speed challenges and doppelgänger races, which has you racing against the 'other' Sonic. Considering modern Sonic has homing attacks and the such, the odds are always stacked against his older, squatter counterpart - which is half the fun.

The final level I saw was a boss, a scrap with Eggman in his Death Egg robot form - a similar altercation as seen at the end of Sonic 2 and Sonic 4: Episode 1, except with a twist. Eggman will still attack by firing one of his robo-arms at Sonic, at which point you're able to get behind him and attack his exposed underbelly. Do this a couple of times, and things change. The screen becomes littered with platforms, which you'll have to use to position yourself in front of several bombs. You're using yourself as bait, essentially, drawing Eggman's attacks to the bombs, then climbing up the robotic limb in his stunned state and whacking him on the bonce.

It's fair to say that Sonic fans have had their fair share of disappointment over the years, but - without building up hopes too much - Generations looks set to be a huge sorry, thank you, and 'Sonic's still got it' rolled into one.

Sonic Generations is due for release for Xbox 360, PlayStation 3 on November 4. A 3DS game is also in the works.