Upgraded to Snow Leopard (Mac OS X 10.6) this weekend and so far all seems to be going smoothly, in fact the entire upgrade operation seems to be much smoother than any previous upgrade.
Only problems I encountered so far are MySQL not working (haven't had time to look into that yet, could be a very simple fix) and PHP throwing up a warning notice about timezone support. Seems like the new PHP wants you to explicitly set a default timezone when calculating stuff using the date() function. No biggy, look in /etc and find php.ini.default, open it up in your favorite editor (BBEdit work great, but sudo pico /etc/php.ini.default works as well) and look for the line that says
Add your preferred timezone, like so:
date.timezone = "Europe/Berlin"
, see the manual
for supported zones/cities.
After you're done with that rename the file using
sudo mv php.ini.default php.ini
and restart your computer.