I haven't tried leopard yet, and I'm not gonna upgrade to it until Apple fixes most of the bugs that some people are having.
The thing that makes OS X great is the awesome user experience. But nothing ruins that more than an application crash, or worse, a system crash. Until an update comes out that leads to a huge drop in people complaining that they're getting crashes, then I'm not switching.
I'm on OS 10.3.9 and safari crashes quite frequently, but other than that, none of my other applications ever unexpectedly quit, and I NEVER get kernel panics. Until 10.5 can match that level of stability then I'm not gonna upgrade.
So I'm hoping for a huge pile of bug fixes.