Our group has ventured to the game Payday: The Heist. This game, in my opinion, is pretty awesome. The graphics are good, the music is great, most importantly it plays great.
Quite a bit from a straight up FPS this game has RPG progression mixed in with the equipment unlocks. Three trees with a total of 144 levels to unlock.
The four player co-op experience is a must as the AI is lacking in certain areas. The AI won't do objectives, won't carry objects. The will however help you up, cover you and follow you. Unfortunately those things can't be expected from human players all the time.
If you can, I would recommend picking this up in a four pack for you and your friends.