iBook G4
1.33 GHz PPC G4, 133 MHz FSB, 512 KB L2
768 MB DDR RAM, 60 GB 4200 RPM HDD
Panther, upgraded to Leopard
Leopard is generally fine. Quick Look is slow quite a bit of the time. Thumbnail generation is fine (as fast as with Panther). Finder is fine but sometimes slow. Boot time is ~2 minutes. Average CPU usage is anywhere from 5%~10% to 30%~40%, depending on the applications open and tasks I'm doing.
Safari works well (when it's all in RAM), even with 20~40 tabs open. Scrolling is fairly snappy.
Video playing: Playing a YouTube video makes the CPU go close to or to 100%. YouTube-quality iTunes videos make the CPU go up to 50%~60% or so. When I pause one of those, playing it from pause makes the video stutter and skip heavily for several seconds to a minute with the CPU at 100% before playing normally. Other iTunes usage (version 8) is good. Some iTunes videos will stutter or almost stutter while playing.
iPhoto 4 isn't quite as fast as I want it to be, especially when creating a new album, and at the end of importing photos. The CPU hits 100% for a minute or so after the last photo, then finishes the import.
iWork '08 is slow at opening, saving, zooming, template applying, and just about anything that isn't basic. But it's far from unusable.
The real slowdowns are caused by lack of memory (heavy virtual memory usage), and these are more pronounced with Leopard than with Panther. I agree with the posters who said enough RAM is essential.