I thought that it was a good and fun game to play through in co-op. I've always tried it online which can also be a lot of fun, but unfortunately also pretty repetetive. It just seemed like a race for the best weapons and only really worked when you were carefully sneaking around behind cover. When people begin playing this game in a Halo style, then it becomes rather boring IMO.
As for the story in the game, I thought that it was below average. Sure it was cool with all the fighting in a semi destroyed world, but it just lacked something. The whole resonator plot was weird, and what followed it didn't make much sense.
I didn't care much for the boss fights either. I think that the best moments in the game where when you were fighting from cover to cover with a lot of regular enemies, and I also found it interesting to deal with the Berzerkers.