Not a singleplayer game.
The first thing that struck me when i installed and played the game was the lack of shadows. In their place were large, black squares. I hate graphical bugs (However, this might be only on my computer. Just a heads up). After playing for a while (trying to ignore the squares), I found out something about the singleplayer. It is really, really boring. Its hard to explain, but around level 15 the game begins to get tedious, mostly due to the mission types. There are three basic missions: Go and activate x of y, go and kill x or y, and go and gather x of y. Thats essentially it, besides a few bos battles and story missions. Yes, the guns (which are randomly generated and can be found in gun crates, or dropped my enemies) add a little spice, but besides from that the solo experience is pretty boring.
So I decide to go and try the online co-op. After installing the required GameSpy arcade, I joined a game...and was surprised by how much better the game was. The co-op is essentially the singleplayer with multiple players (did I mention that you play alone in singleplayer? No AI allies), but with an interesting touch. As more players join (the maximum is four players, and yes, more than one player can play as the same class), the enemies get harder, and the loot gets better. It adds a fun touch to the game. I have done one particular mission at least twenty times, and each time was a little different.
If you are looking for a great singleplayer RPS game, get a game like Fallout 3. If you want a good competitive multiplayer game, get anything other than this game. But if you are looking for a great co-op experience, by all means get Borderlands.
P.S. The story is very shallow, dont get this game if you want an engaging story. However, the artistic graphics and interesting NPCs help pull you in.