A comic-based game that rocks.
Most of the game consists of running through interesting locations, fighting Sentinels, Morlocks, Brotherhood mutants and humans as a team of four X-men. You can switch between any of the X-men in your party on the fly, but I would recommend playing with as many human players as you can. Up to three other players can jump in and control your other party members. I played through the whole game with a friend and it was awesome.
Despite the relative simplicity, there is a lot to this game. There are plenty of abilities to upgrade for each mutant, and a lot of additional content like danger room discs (which grant access to new danger room missions where you can boost your stats and get sweet gear) and comic books (which I think unlock art and boost stats for certain characters).
All of this stuff is good, but to be honest, I hardly bothered with any of it. In fact, I didn't pay very much attention to the story either. I left the room and let my friend play all of the mandatory 1-player sections and pretty much skipped the dialogue all together. The reason I did this is because all I cared about was the gameplay.
The gameplay is really the highlight of X-men Legends, in my opinion. Beating up on bad guys and combining mutant powers for big combos is just so satisfying. Also, since there's a lot of destructible stuff in the environments, you could, for example, kill an enemy with a single optic blast from Cyclops and blow up the wall behind him. Throw in moments where you team up with other mutants for a huge attack, destroying multiple enemies and pretty much everything in the environment and your bad-assness goes off the charts.
X-men Legends is a great game, especially if you can play it multiplayer. It's not the deepest game of it's kind, but it's still complex enough to make it interesting. The story, the items and the extras are all great, I'm sure, but I was too busy kicking butt to notice.