I had fun with Borderlands. Plus, you really ought to try it multiplayer. It's more fun that way, and certain painful quests or "That One Boss" fights are much easier to complete with teamwork.
My brother and I dueled quite a few times inside the Skag arena (not Skagzilla; after completing that quest), and it made things rather interesting, seeing as we were still able to get mauled by them, and we kept on killing each other in the dueling arena dome that appears just as the skags were respawning and interfering our duels. It got pretty entertaining.
Not to forget, my brother playing as Mordecai and me playing as Lilith, him 2 levels above me, and me equipped with a shock shotgun (which I have some skill in and a x1 chance to shock), and him relying on his sniper skills and Bloodwing. It was a surprise to him that I was able to kick his ass from 2 levels below.

Then again, I also knew how to snipe with a shotgun, and the shock ability improved my odds of causing harm. Plus, bunny-hopping is a bad idea when your opponent is armed with a spread-fire weapon. I could easily calculate where he was going to land, waited, and fired (with of course several milliseconds preemptive fire to account for potential lag).
A good duel/multiplayer arena tip, if in a duel, and you're playing as Lilith, watch out for Bloodwing (if your opponent is playing as Mordecai) and just before he swoops in to kill you off, phase out and you'll be immune to Bloodwing for a short time and you'll have an opening to finish off Mordecai, provided you KO'd his shields first. You might get lucky and kill them off from up to 5 levels below easily. If you have some perks added to your melee, those become useful as well. Nothing more fun than stunning your opponent and blowing out their skull at point-blank. Phasing out, notwithstanding, being especially useful here.