So, I just finished Deadpool, and I can tell you this: only grab it if you're interested in Deadpool as a character.
The game is suprisingly fun to play, but it's really short, and can be finished in no time.
Still, it has Deadpools typical completely batshit insane hijinks, such as commenting on every kill ("Aww, that's my favorite bullet! Can ya bring it back?"), telling the player off if you get hit ("Give the controller to someone who CAN play") and a complete scene in which you have to shoot baddies in a big foot.
Some cameos by X-men characters, but nothing noteworthy except for Cable.
So, in short. It's a nice hack and slash game, made great by Deadpools hilarious antics. Like I said, only get it if you like Deadpool as a character.