I just maxed out my character last night. With the DLC's, your max level is 69, the standard game maxes at level 50.
Just a word of warning if you have the Secret Armory of General Knoxx DLC. The quest to blow up the armory is basically the end of the storyline quests, much like entering the vault in the main game, so after completing these quests on your second playthrough, Borderlands will turn into a world of death with all enemies continually spawning a level above yours.
This is not such a big deal in the main game because you've more than likely completed all the side quests before entering the vault, (If you're a completionist like me...) but with the Secret Armory, there are quests that you will not be given until you blow up the armory. This means that all further quests will have the 'Tough' rating because all enemies now outlevel you. It's just a bit of an annoyance with common enemies, but can be a brick wall with some of the boss enemies.
I don't think I'll be able to complete my second playthrough because although I have really nice weapons by this point, I just can't do enough damage to kill a particular boss before more healing units spawn and restore him back to full health. :black_eye:
On the plus side, I only need 3 more Xbox achievements to have all 80.
Unfortunately, two of those (after which will give me the third) will take close to two hours each to complete, so I'm less than enthused to get them.