3 days later I am having much better results.
In a nutshell I'd say this, 'upgrade in place' is likely to result in problems. I personally think its a flaw that in 2007 I have to say this: backup your system fully, then do wipe and install. If that makes you skiddish, backup and do an archive and install which should preserve your applications, documents and settings.

Once installed, I'd ballpark that 90% of things work, and work very well. While OS X 10.5 isn't likely to make a huge change in how you use and interact with your computer (something I think a major OS upgrade should do) it is a very solid and worthwile upgrade. I am frankly amazed that for $200 (family pack) you can upgrade all (5) of your computers to something that is a fairly complete system ... compare that to Windows Vista Ultimate at $300-$400 which is far from being as complete as OS X and is only valid (locked) to one PC.

Now that said, its the 10% of things that are broken that may really ruin the experience for some users. Both Parallels and VMware rely on kernel extensions which are likely to be broken (understand VMware has a beta that works with Leopard). My guess is thats is the power users (IE Geeks) who will likely run into the most problems as most day-to-day applications seem to be running quite well.

Thats my weekend-with-Leopard recap