Outstanding for co-op. Not so great for single player. I actually didn't mind it but I think the best way to describe it is it's basically first person Diablo II with guns. If you can't stand the thought of playing D2 by yourself, then you won't like Borderlands in single player either.
Length is decent, you get 2 playthroughs (2.5 actually), but like zerocool said, a lot of the quests are very repetitive. Lots of "go here and kill X" or "collect 20 of some item." Not much story but if you're playing with friends it doesn't really matter.
It isn't dark at all, even in caves or at night. The art style is cartoony and colorful with thick black outlines of everything, giving it that cel shaded look.
Again, if you just think of it as Diablo II with guns it makes sense. Lots of random loot drops, occasionally you get something good, you have character classes and skill trees, enemies are basically all the same, but it's still fun.