It's a fun game - I wish they made more games with co-op play in mind. I've only had time to play it once so far but I downloaded it the other day and I played over hamachi with a couple of friends. We like it enough that we decided to all go buy it so we can test out playing with pick up groups.
I agree that it's really fun to play with others. There isn't much plot to it, just pick up a gun....blast everything in sight....pick up better gun...blast stronger enemies. Its kind of like a mix between Halo and Diablo2 (always trying to pick up stronger weapons and shields). I've been playing with the soldier. It still brings a smile to my face every time I get a critical hit or take an enemies head off and my character yells "critical, bitch!".
This was a good game. I just traded it in when COD came out. Now I have LFD2 and Assissins Creed 2 so there will be not time for that. I might get it again after it drops in price just to get some of the perks I missed.