First of all, this isn't so much a review as it is a very simple comparison. My old PowerBook G4 laptop was nice -- even with only a 1.5ghz PowerPC processor it was quite snappy once it managed to get past the startup process. I'm sure the 1280MB RAM helped with the speed though, as Mac's seem to benefit a lot from extra memory.
My new MacBook Pro is blazing fast, and bumping the RAM to 4GB really made it fly. Now my laptop has 4x more memory, a much better CPU, and a much better graphics card than my desktop! I'm starting to wonder why I even have a desktop...
Despite their huge differences under the hood, these two laptops looked very similar side by side (the MacBook Pro is on the right):
Although almost everything else is smaller on the MacBook Pro, the battery and power supply are much bigger. Thankfully, the new MacBook Pro's don't seem to suffer from the annoying warped/bowed lid, as is partially visible on the picture up top (PowerBook is on the left).
I sold the PowerBook to my brothers' girlfriend and I'm writing most of this review from memory. Because of this I'm unable to honestly compare things like weight and differences between the keyboard/trackpad. But I can tell you one thing for sure: At the price you can pick up a used PowerBook for (dirt cheap), they're nice little laptops.
The PowerBook is as stable as any Mac and it's great for doing basic things. If you're a techie who needs a cheap but elegant *nix based system or if you're just wanting to explore the world of OS X, a used PowerBook is a great way to go. Keep in mind though, if it doesn't have an Intel processor (the PowerBooks have G4 PowerPC processors) you won't be able to run Microsoft Windows using VMWare or Parallels.