FYI- Deadpool mysteriously disappeared from Steam this morning, right in the middle of a flash sale. (And XBL and PSN too.) There are strong rumors right now that Activision is losing the rights to the character at the end of the year. If you want the game, it's still available at Amazon ($9.99) and Gamefly ($7.99) but who knows for how much longer. Console copies will still be floating around, but once again, no one knows for how long.
Yes, it activates on Steam. And, just as an FYI, in these cases, once a game is in your library, Steam will let you keep downloading it. But no one else can pick it up beyond then. You can still use the code to activate it (I just did this with the DLC) but once again, not sure for how long.
It was a decent game, if a bit repetitive. The voice work was awesome though.