I bit the bullet and installed OSX 10.5 "Leopard" on Friday. I first did a full backup, then did a clean install and used the Migration Assistant to transfer my user folder, settings and applications from the backup. So far it seems to have worked well, with the exception of several applications (all of which were Mail.app extensions such as MailTags, GPGMail, GrowlMail, etc.)
I did have one annoying issue that I just recently have resolved. I noticed that a process ATSServer was continually eating up lots of my CPU cycles. I eventually tried killing it but it would respawn and resume eating up CPU cycles doing whatever it was doing. After a bit of searching I found that this process is related to Apple Type Services and the problem is typically due to corrupt fonts or a damaged cache file. Long story short, this wasn't the issue and none of the provided fixes helped.
After reading through my log files I found a mention of a permission error, so I decided to run Disk Utility and repair the file permissions on my volume. Then I reboot and the problem seems to be gone.
Hopefully this will help anyone else having this problem.