I used regular until about a month ago on PCs, until I played a friend's machine for a couple of minutes which he'd left on inverted...and I couldn't manage to revert back to normal. Initially, I was inverted, but switched a few years back.
I started on regular when mouse/free-look first-person shooters started popping up like rabbits in the mid-90s, switched to reverse around the Quake II/Dark Forces II in the late 90s, and then switched back to regular during the last couple of years for some reason.
Didn't we just have this argument a week ago? At any rate, the whole debate seems fairly stupid. I don't think that either setup gives you an advantage, it is most likely just a matter of preference. Though it might be interesting to see what the world's best Quake and UT players use......
Regular in everything, for as long as I can remember, except Metroid Prime where I use the default inverted. Kinda weird. I changed a few times though (in MP) and realized a) you adapt really fast b) it doesn't matter what you use.
Something snapped in me a little while ago. I've been playing regular for ever, and then I just decided to give inverted another go. Now I can hardly control my character if it's not inverted.