OK, I've been using Leopard for a few days now. My first impressions are that it is awesome. I've run into no compatibility issues thus far. I like the look which clearly Apple has focused on as much as function.
One big noticeable improvement is the speed. I'm using a PowerBook G4 (1.5 GHz with 768 MB RAM). By most standards, that is getting old. Leopard has made my machine run a lot faster and smoother.
The only issue I have noticed (and I'm not sure to what extent Leopard is to blame) is in syncing. When syncing my computer to a server, it doubled some of my files. One had a "-local" suffix and the other had a "-network" suffix. I'm still trying to figure that one out.
I still wish Apple would do something about Appletalk, but maybe they can't just dump it. I don't know. It's just an annoyance.
Is it worth the cost? Well, for me it didn't cost a thing. My employer covered that. Would I, personally, shell out the $130 to update? Maybe. The change from 10.4 to 10.5 is not too drastic. If I was still running 10.3 on an iBook, I would do it in a minute.