My main is sitting comfortably at 70, running instances, edging up her tailoring skill, and slowly accruing cash for the epic flyer. By this point, in fact, I've brought my second character to 60, my Rogue, and started her in Outland. It's just as much of a blast as I remember it being the first time, months ago, although of course it's a little less fresh. I remember everything being so big and bewildering back then; now, knowing where everything is and what all the quests are, it's very much more manageable. It's still challenging, though, since the mobs behave a bit differently than I'm used to from the old world. If you've gotten a second (or third, or...) character to Outland level, how have you found it the second time through?
Also, what have you found to be a good level to run through that Dark Portal and start questing? I originally went at 58, but after doing a few quests decided I'd better go back to the Plaguelands and get to 60 first, for more effective questing. Still, I'm glad I did the first few quests, since they got me huge upgrades in main- and off-hand weapons, and a piece or two of armor. If I become crazy enough to get a third character out of Azeroth, I'll probably do it the same way -- go in for a few easy pieces of vital gear, and come back out to get to 60. What would you do?