I upgraded from 10.5.8 on my 2 month-old 13 inch MBP and I think I have not had any problems. I say "I think" because I can't really tell I've installed a new OS X. It's just the same as before. I see no improvements in speed. It's like I haven't really installed an "improved" OS X.
I understand there are "under the hood improvements, blah blah blah" but if you replaced all the innards of a BMW with Bugatti innards, including engine, but afterwards your "BMW" still runs like a BMW, was it really worth it?