I quite enjoyed Borderlands. Unfortunately though, my single player experience was far better than my online experience. The game won't pick up sound from my integrated mic for shit even on the highest sensitivity. It's a horrible pain in the sack to find a server with minimal lag. If you happen to join a game where someone just completed a main quest you were after, you're ineligible for the rest. It feels worse to lose loot you needed than in Diablo.
If you find the right server with the right people then it could be fun. Ultimately, the online experience doesn't offer much more than the single player, which sucks because GB tries to influence online play by making the drops better, so you're kind of shafting yourself by playing offline a little bit.