I played through Borderlands almost exclusively on singleplayer because I have the same problem with friends that Yahtzee does. My brother and I did co-op briefly a few times (after getting through the hellhole that is the online system) but it just didn't work because we have completely different playstyles. I'm the type that enjoys hanging back, scoping out the whole battlefield and picking guys off with a sniper at a leisurely pace, while he's the kind that enjoys running in with a shotgun/SMG and blasting anything that moves. I can't imagine what the game would be like with four people, trying to coordinate anything would be akin to pulling teeth. Also, you get into fights over loot pretty regularly.
Regardless, I had fun, and got a decent way into my second playthrough before Mass Effect 2 came along and sucked all my gaming time into its sweet, sweet depths. I enjoy a touch of easy mode (nothing annoys me more than having to level up to be able to beat a mission), but it was challenging enough to keep me engaged. The art design is fantastic, there's a lot more enemy variation and strategy than Yahtzee lets on, and there's about as much mission variation as an FPS will allow (kill something, collect a number of somethings while killing something, find someone while killing somethings, and a combination of the above). Really, the only thing the game doesn't have is puzzles, which is probably what really annoyed our favorite adventure-game-lovin' critic more than any of his other complaints.
There's still a number of glitches and bugs in the game, but overall it's a solid B- as far as games go. You'll have fun for a while, maybe even enough to finish the game once or twice, but it's not the sort of thing you'll be revisiting a year from now.