Rent it first
Recently I rented Ultimate Spiderman, and I thought it lived up to it's we'll...average expectations. First, let me start off by saying that I am a comic book fan, although I have never read the Ultimate Spiderman comics.
Gameplay, is fun, simple, very easy to get into, a game that you could just pick up and it would take you about a minute or so to figure out. What I personally found the most fun (the same with the Spiderman 2 movie game) is just swinging around NYC scince they made the game map nice and big, and explorable. Although i'm not so sure I like the art of the buldings and enviroment as much as the Spiderman 2 move based game, its still very fun.
As far as the story goes, you would be dissapointed if you bought this game at its newly released retail price. And thats because I beat the game in about 6-7 hours. But not only the length, but from my view they got the characters terribly wrong. I'm not going to get into comics now, but as an opinion, I didn't like the villans at all.
Graphics and sound are all real good, not great but good. In conclusion rent it first, and if you enjoy just webbing around NYC a whole lot, wait until it comes down in price a little bit, then buyt it.