Borderlands is not a single player game. Its also not an MMO. What it is is the first successful attempt at recreating the Diablo experience. The amount of similarities in play style are stagering, the only major thing that is missing is the lack of randomly generated maps.
Coop is great because it doesn't take a lot of overt team work to make a good team. I have found more often then not that groups will quickly start working together without ever having to say anything to each other. This, if you remember, is almost exactly how Diablo played.
The itemization is fantastic, and the FPS action is remarkably good. The enemies are well thought out and interesting, although the variety is slightly more limited than I would like. The game plays fast and very very fun.
The only issues I have with the game come down to the technical issues surrounding the PC port. Its not bugs/crashes, but more microphone and menu implementation.
Seriously, this game is an absolute must have for anyone that enjoyed Diablo. Seriously. I'm super cereal.
Edit:
Oh yeah, and its funny as hell:
It has some of the best voice acting I have ever heard (remember that the context is humor). I can't find a good audio clip of Scooter (by far the funniest, but here's his voice saying something not that funny
[Youtube=http://www.youtube.com/watch?v=fcgNVhgOEdU][/Youtube]
Now imagine that guy saying this stuff
"Hey go find my friend Lucky, and by friend I mean scumbag asshole who messed up all my mommas girl parts"
"Thanks for fixing the catch-a-ride station. That thing was more messed up than my momma's girl parts. Thanks for taking a poke at that. The catch-a-ride station, not my mom."
"Hey, Scooter says don't walk, drive! My daddy always said wheels were better than heels. Uh, he was paralyzed, though. Anyway, don't walk, drive!"