I’ve not played much of Ultimate Alliance yet, but I think I can explain, with a quick quote from Wikipedia, why it is a great game:

An IGN playtest featured a preview of various heroes’ alternate costumes, where Beta Ray Bill was confirmed as a costume for Thor, and that Scarlet Spider would be one of Spider-Man’s available costumes. In interviews with the developer, it was remarked that Spider-Man will also have the black Symbiote costume. It was also noted that all the characters will have at least four skins. Many of these costumes have been released on the VIP site.

Emphasis mine.

Excuse me while I go dig up a cheat code. (Really, I’m not wasting time unlocking that costume when I could be playing with it right now!)

[Update: Do not use cheat codes if you like the achievements on Xbox Live. For obvious reasons, using cheat codes disables your ability to earn achievements. For unobvious reasons, it is absolutely impossible to turn off a cheat code on Ultimate Alliance. I have deleted my save games, deleted my gamer profile, and done everything I could think of short of reformatting my Xbox 360 hard drive. The extra costumes I unlocked are still there, and I can't earn achievements.]