Yesterday I used my friend's 12" PBG4 and I was impressed by the quality. The tactile feel of the PBG4 keyboards is so much better and smoother than the classic MBPs, which is still better than the current chiclet keyboards. I thought I was crazy for thinking that my old 15" PBG4 had a better keyboard than my MBP, but the more PowerBooks and Macbook Pros I use it just confirm it.
I always get the impression that Mac OS X feels more "solid" when its running on PowerPC processors. I don't know what it is, the PPC and Intel versions have a different feel to them.