Welll.....
I just got a new 12" PB, super drive, the works, and I love it. I had aimed for th emost portable I can get to take to hotels, vacations, and general slouching at home. The keyboard is the same as on all of them, my friend has a 15" and to me, it's just too big, because its the size of most 17" PC laptops due to the huge speakers they stuck on the sides of the PB, and the 17", ugh.. Try fitting that in your lap or on the tray table on a plane!
All in all, no regrets, I had a IBM T41 with a 14" display, loved it but I can't get enough of OS X, love it ^_^
If you wan tmobility and small size, 12" is the way to go.