If you have a friend or three to play it with, I highly recommend it. My friend and I went through it co-op from start to finish and had a great time. Took just over 20 hours, so it's pretty long.
The only parts that drag it down are the mansion bits, where only one player can play and it's basically just searching for a particular mutant.
I wouldn't buy it, since there's almost no replay value (only new costumes, after you beat it). It's well worth a week-long rental.
It's an RPG with real-time fighting. It's a good mix.
I bought it release day, and had some great fun with it, BUT only when I had 2 friends playing with me. Single player is boring, dragged out, and your computer controlled teammates are real dumbasses.
It is great fun timing combo's, planning the skill tree so your group has the best attributes and working together. Keep in mind it WILL cause a lot of strife when you have a friend who insists on falling into every damn pit and draining all your cash resserecting him, or believes COlossus is an invincible tank and can gulp down 12-15 potions in a 45 second period, all the while sucking it up.
But I'm not bitter.
All in all, it was worth the 25 hours or so we put into it, as we had a blast, but with zero replay value, I traded it in towards a Halo 2 pre-order.
I think this game is a lot of fun. I agree with previous posts though that say it is most fun when played with other people. It's not the same game in single player.
I was excited about this until I read the reviews. I heard "X-Men RPG" originally, and expected something like KOTOR with mutants. Turns out it's more Hunter: The Reckoning with mutants... Not really what I was interested in.