I upgraded to Mac OS X 10.5, more commonly know as Leopard back in November shortly after it was released. The upgrade was not all that painful. There were a few applications that stopped working. There were a few features moved around. This experience is consistent with every OS upgrade that I have ever done.
For some great help with Leopard and your applications, the PSU Mac System Admins wiki has a great list. Have a look. Maybe even add a tip if you have one.
Since I ran the actual upgrade, there were a few things that stopped working. Since clean installs are usually recommended, most of my problems were self-inflicted. Your mileage may vary. My own list of program glitches included the following:
- Parallels 3 which released an update for Leopard shortly after I installed.
- Norton AV 10.x Corporate's LiveUpdate was crashing. The FAQs linked to from the Mac Sysadmins page fixed that problem.
- Cisco VPN Client failed to connect. Reinstalling the version 4.9 fixed the problem.
- Wireshark porting has changed due to the changes in the Apple implemetation of X11.
Overall that wasn't much of a list of problems. No big reasons not to upgrade as long as your main apps can go with you.

