I personally don't like it.
Feels too much like a RPG for my liking (reminds me of STALKER, maybe Fallout 3 too if I had played it), and the kind of cartoonish graphics I'm not too fond of either.
Though I too have a so-called evaluation version, so no co-op but I might try co-op with my brother and dad. However I somehow doubt it'll change my mind enough to buy it.
And right now with my semester starting to come to a close, I don't have much time to game, nor have I had much time at all this semester. Being at school until 6:30 and once I get home and have dinner its around 8:00...it sucks

Then I have to do homework/study and get some sleep in there to be up for 6AM the next morning and start again.
Then I'm assuming I'll be working 9-5 in the new year, but at least when I get home it'll be all free time

So I'll have a chance to game then, but I don't think it'll be Borderlands. Probably hit up L4D2.