I've been running leopard on a powerbook G4 (1.5Ghz) since release day (fresh install).

7 Crashes in safari
9 Crashes in quicktime player
1 iTunes crash
1 iPhoto crash
1 Xcode crash

And that's about it. I can't ever recall seeing mail crash under leopard (or tiger) and I don't think I've ever seen finder crash either.

Adrian