While playing co-op certainly makes the game easier, it's still a very enjoyable game for it's single player content. Being a FPS-Diablo-esque dungeon crawler, the game offers plenty to do, and achieve on your own. Plus you don't
have to share in the loot
Sharing the loot isn't a problem in Borderlands, though, because the loot generally sucks. I was highly disappointed with the special alien weapons (I can't remember what they're called). The only weapon that was really worth it was the Hellfire SMG or
whatever my friends called it. See how much I really paid attention to the game?
I got stuck EXTREMELY early on... I don't know if I'm the wrong class, or using the wrong weapons, or the wrong tactics, but Borderlands picked a bad time to be difficult. I don't think I've played it at all since last year, and I never managed to find someone
to play it with. I wouldn't recommend going it alone at all. And I really wanted to like it too, now it's just something I'm gonna pick up again one of these days. (Just like Tony Hawk's Project 8)
You can still play and have fun in Borderlands if you really enjoy stat-crunching and are typically a solitary gamer. However, Borderlands really begins to shine when you play multiplayer. The game's the best when it's full and every character is playing
as a different class.