I certainly won't talk anyone out of the PowerBook, but I'll stick up for the even the low end iBooks (which I have one).
My 12" iBook has become my new favorite computer of all time. It used to be my iMac 400. By favorite, I mean in a "VW Bug" sort of way. Sure, it's not the most powerful by a long shot. It may even have a few missing features or "issues", but doggone if the little thing doesn't deliver what I ask it to do.
One thing I would remind a purchaser is to think about pixel resolution. I was going to get the 15" iBook, but then I realized that it also maxed out at 1024x768 (same as the 12"), which basically meant there was no pixel quantity benefit to the larger screen. I could read the 12" just fine, so I went with that. I also don't need my iBook for demanding applications in general, though I did edit an onsite trade show video with it and it did just fine.
As always, I'd say it depends on a user's needs. IMO--none of the laptops, except the highest end models, really impress me in terms of performance. I did some in store tests and found the models to get progressively faster asone would expect, but only marginally so until you get to the big boys.
For my needs, I just couldn't convince myself that the PowerBooks were worth it. And I was right. For someone else, it could easily be worth it to pop for more speed and features.