Hey Yotabyte,
I don't have much to report, as I've only had it a few hours! But to me, it just feels a lot more solid and like a new, fancy computer. Bear in mind that when I sold my black G3 wallstreet and got the iBook, I was really taken by the small size of the iBook. So I think part of it is that I just wanted something that felt new - the 12" seemed so much like my iBook and yet different... and I'm still quite nostalgic for the iBook, I really love it.
The 15" seems sturdy, beautiful, luxurious... and big. It's really not much heavier, but definitely bulkier. I feel a bit nervous taking it out of the bag with one hand, for example. But as long as I can get decent battery life out of it, I think I really prefer it. When the 12" arrived I was trying to make myself like it. When I opened this one up, I was oohing and aahing. The backlit keyboard is so cool! (And I may be part of the 3% who actually needs it as I gig with the thing on occasion). The screen is beautiful! And as I am (a) a composer and media artist, and (b) writing my dissertation on film and music, I think it will make a big difference having that big beautiful screen.
I must say, though, that I think for many people a 12" with an external monitor would be a good choice. If you get the 1.33 powerbook, the processor is exactly the same, and hooking it up with a 17" or bigger monitor at home, you'd actually get a lot MORE screen real estate. It would have actually made my desk a lot neater, as the pbook keyboard takes up a lot of desk space. I think my problem is that I knew that that system made sense, but I really WANTED the 15"! I have always been somewhat anti-desktop for some reason, it just seems bigger and bulkier and messier.
So - I am happy. Very happy. Let me know if you have any other questions about it. I'm now going to calibrate and cross my fingers for good battery time - it seems to be fairly hit or miss.