I just installed the $29 Snow Leopard upgrade. Only thing that I noticed is that my old Safari icon stayed on the dock, and it didn’t work– stated that I had to run the newer version. Easily fixed though– I just removed the old from the dock, went to the Applications folder, started the updated one and chose ‘Options-> Keep in Dock’.
No major disasters, but no major every-day improvements that I noticed either.
Check out CNet’s review for details that I have yet to dig into.