It gets boring fast. Having your Deadliest Warrior's arm sliced off and watching him carry on as if nothing had happened is funny the first few times, but then it becomes dull and commonplace. Heads will keep rolling, but in terms of game design it's a bit of a cheap parlour trick - offering geysers of blood to make up for a lack of quality mechanics.
Single-player has you crack the skulls of all eight Deadliest Warriors; A mirror match half-way through gives you a bonus costume, and the occasional bonus stage has you hacking down pig carcasses or fighting in an instant-decapitation round. Succeeding in these gives you a new weapon that has roughly the same stats as the previous weapon, and that's the only real impetus for progression.
Finishing the single-player as every character does unlock the survival mode, though, which is actually even less exciting than it sounds. The only reason for doing this is getting the associated Achievement, but you probably won't bother.
Like all fighting games, Deadliest Warrior is at its best when you're playing with someone else - two Deadliest Warriors locked in a gladiatorial duel to the death and also the best of three rounds. You better keep it local though: the online netcode turns the game into a slideshow, and one time my head seemed to pop off all by itself. And in the game, too.
You'll also have to watch it load, which takes almost as long as each fight. Loading screens also have a mishmash of useless tutorials - they might as well just write 'don't die' on the screen in bloody letters - and the odd fact about each fighter that's painfully obvious and probably lifted from the top of a Wikipedia page: now that's edutainment. It also plays the same audio flourish over and over and over and over and please kill me and over again.
The Deadliest Warrior only costs 800 Microsoft points, but you could definitely spend them on something better. It's fun for all of 20 minutes, and then you'll either be ready to delete it forever or will have moved on to a better fighting game. Either way, don't bother. You'll have more fun watching the brilliant-but-awful TV show and then playing something - anything - else.