After careful consideration of the various available models, I chose the iBook. More specifically, the small model with the 12" screen.
The 12" model was a most important to me, because I want a laptop small enough and light enough to carry with me wherever I go. Small enough to be useful on my lap while a passenger in a car, bus or airplane. Although I'd prefer a higher resolution display, that is not available without moving up to a 15" or 17" PowerBook, which is both larger and more expensive than what I'd like.
I bought my iBook with a few upgrades. An 80G hard drive (for an extra $100) and 1.5GB of memory (for an extra $300). I could have upgraded the memory myself for $200, but I decided that $100 is not an unfair price to pay to have the memory installed and tested at the factory. And if there's ever a problem with the computer, Apple won't be able to tell me that it was because of third-party memory.
Anyway, I haven't had too much time to do too much with it yet, since it only arrived today, but so far everything has been working as expected. As expected, I was not able to copy my Emacs installation from my other computer, forcing me to download sources and compile it myself. This went smoothly, since I've had to do this several times before on my other Mac. Other applications (Thunderbird, Firefox, Microsoft Office, etc.) all installed without problem.