You can add me to the list! I've never connected my 12" PB to an external monitor, and have never wanted to. I use it for hours on end daily, and don't wish it had a larger screen, and have never experienced eye strain as a result of using it. I have only booted up the old 15" Windows laptop maybe 3 times since getting the Powerbook, and only stay on the Windows machine as long as it takes to convert my files to PDF and get them off of there and onto my PB. My preference for my 12" PB is so high that when my husband's at work, I don't use his available 15" PB, since I prefer my machine instead. I use mainly iTunes, Mail, Pages, Safari, and iCal. I don't do any artwork, so I haven't used the machine with any type of Photoshop program. So for me, no regrets, and if I had the opportunity to go back in time, I'd still pick the 12" PB.
It's a very personal decision, though--my husband definitely likes his 15" PB, likes connecting it to an external monitor (and the TV), and drools over the large ACD screens at the store. So while the 12" was right for me, I don't think he would have been as happy if he would have gone smaller. Have you had the opportunity to try both iBooks? Is your consideration of the 12" primarily because you are looking for something small and ultraportable, or primarily because of the price?