Well, definitely for a day-to-day, casual or "consumer" customer the iBook wins, hands down. I wouldn't suggest anything BUT an iBook for a buddy or family member interested in surfing/e-mail/iApps usage, or a student doing writing and research.
A PowerBook, in those situations, would be a bit overkill AND an unnecessary added expense (with MAYBE the exception of the 12" model...but the 15" and 17"? No way!).
I've been responsible - in the past year alone - for five iBook sales (three since they went to a G4), so I'm a fan and definitely appreciate the model!
In fact, ANOTHER friend of mine is ordering a new 12" iBook next week, after months of picking my brain and learning about Macs. She's only dabbled in Windows up 'til now, but when I showed her iChat, iTunes, iPhoto and told her about .Mac, iSight and OS X in general, that got her interest. She's buying the iBook and an iSight and I'll surprise her with a .Mac account, just as a nice added bonus.