Well, the "service pack" is finally out, and upgrade went smoothly on the Mini I use at work. Took about 20 minutes, and was a little sluggish for a few minutes as it reindexed some things. Only compatibility issue that I have is that iStat Menus went away. Sadly I can't use the Exchange support built in until the server is migrated to 2007, so no idea how well that works.

I found this site handy for compatibility issues. Apple also has their own.

Overall, $49 for a 5 pack license to cover all my machines seems reasonable. I'll be curious to see how quickly 3rd party apps adapt some of the newer tech to make them speedier.