The knock on the iBook keyboard is its flexibility, which some might call sponginess. It has some give to it. Maybe this applies more to harder typists, I don't know. The iBook keyboard is a removable piece of plastic that covers the slots for RAM and Airport. Many users report that it can't quite sit level, or that the space bar sits unevenly (higher on one side) no matter what, etc. Some people just don't like the feel of it, but of course this is subjective. Personally, the current PowerBooks' keyboard is the best-feeling keyboard I have ever used, desktop or laptop, but that's just my opinion. Many people get along just fine with their iBook keyboards.
As for the PB being "too expensive," it is certainly more expensive than the iBook, but when you use BTO options to configure them similarly (RAM, HDD, and BT), the difference in $US is only 20% of the PB price. That's not necessarily a huge difference in the context of your larger purchase, and you do get faster CPU/FSB/HDD/GPU, twice the VRAM, and a few other extras that may or may not prove useful. But yes, the iBook is cheaper. If the iBook keyboard doesn't suit you, search for rev. C PBs on closeout; some deals have been reported hereabouts.