I upgraded to leopard on my Powerbook G4 12". I shortly after noticed that programs seem to crash more often than they did in 10.4, but many things run very well. I haven't had anything crash on me yet that created any more than a slight nuisance. I am also aware that my computer is quite a bit past its prime and even though I love it to pieces I recognize that Its becoming obsolete. As such I've learned to accept the occasional crash (which only affects one program at a time) and I've become so addicted to the features in Leopard, quicklook in particular that I could never go back. I love screen sharing too, since i have a desktop too. I also happen to care very much about the asthetic value of leopard. Overall I think I'm happier using leopard than I was using tiger, but I do admit I was a bit dissapointed by apple's work, and If i was only using my brain and not my heart I would be inclined to possibly return to leopard. However, after a few short months it's become natural to me and I wouldn't switch back. I recommend leopard but if you are planning on buying a new mac in the next year, I'd say skip it, because even though it's neat to have it might give you trouble. I also would like to know if people running faster machines (specifically intel machines) are having problems with programs crashing. All in all i think apple will end up fixing these quibbles with updates
so to sum that all up, I love leopard, but i do acknowledge that It is dissapointing in some areas. Id upgrade, but that's me. And I cant remember how i lived without quick look