OK- this is my own opinion after I got my iBook G4...
1- The 14 inch model is awesome! Much better than the 12 inch. It almost has the feel of a macbook. I'd like to have a higher res. display, though- 1024/ 768 seems a tad too big.
2- Leopard runs almost four times faster than Tiger on my old g3, which I gave to my brother. Apps open in 2 or three bounces, unlike my G3, which open stuff in like 6 or 7.
3- Time machine is great on my reformatted 2GB USB Drive. I only backed up my home folder. However, I'd like to know how to get the animated background in the "Time Portal", as my G4 doesn't have it.
4- All the newer apps open faster also (like Safari 5 and iTunes) and Youtube video watching and Flash is almost as good as any newer Mac.
5- DVDs play very well, and all video is very smooth.
6- The Xcode developer tools work great, and (so far) the small apps that I've made are even compatible with Mac OS X 10.7!
Conclusion- Leopard is (by FAR) a 100% better operating system than Tiger, and my G4 is (also by FAR) a 100% better and faster computer.
I think that anyone who has an iBook G4, or any other G4 should definitely get Leopard!
PS- Does anyone know how to get a translucent menu bar, and the Time Portal animation? Or the Dashboard Ripple? Thanks!