I've heard the game is a helluva lot of fun in co-op (I personally have put 150+ hours in single player, but zero in co-op) but cheating/hacking is pretty rampant in random online games. Ideally you'd play with some trusted friends, or maybe with people through this website. Otherwise, you're probably gonna end up with some hacked items that trump anything you could ever find in-game, and thus one of the most fun aspects of the game, which is the slow ramp up of power, with the occasional exciting boost by finding a truly unique/powerful upgrade, will be lost.
It's a GREAT game, but if you're getting it solely for online co-op, be wary of who you play with.