We'd just gotten a replacement for the iBook, a 12-inch G4 when I read rumors of the possible impending release of the 15-inch G4. Fortunately we didn't open the 12-inch and were able to return it for a 15-inch beauty. The 15-inch ended up costing about $400 more than the 12. It has a little less RAM (512 MB instead of 640MB) but much more possibility for expansion. It also has a superdrive instead of the combo drive. I'm not entirely convinced that the superdrive is right for a laptop. On the 12 I have I've liked the rapid speed for burning CD's which I do much more than DVDs.
The keyboard on the 15 is similar to the 12 but not as comfortable for those, who like me have poor typing position. I tend to rest my wrists on the computer too much. This is very comfortable on the 12 given the spacing of the keyboard, case etc. On the 15 it is not as comfortable as the keyboard is further from the edges of the case and it does not allow the wrists to bend down at the edge of the case as the 12 does. The illuminated keyboard seems to be a neat feature. I haven't really used it yet other than to cover the edges of the keyboard and see the lights come on. Read more about 15-Inch Powerbook