I just upgraded yesterday, and while it's not all good news I'm pretty please with the update. I can't really quantify how much faster it feels, but if I had to say, I'd say "An eentsy bit." Just barely noticable. But there's just tons of nips and tucks throughout the system that I found aggravating in Leopard, and I'm glad they're fixed (i.e. the way navigating backward in Icon view in the Leopard Finder would reset to the top of the scroll). Most applications are working just fine, although the ones that were iffy on Leopard (like Office 08) feel even slower in SL, though I don't necessarily blame the OS. I thought I might be aggravated by the new Exposé because all windows tend to become the same size in hierarchy, but the fact that I can see labels (LABELS!) now on all the windows is great, and so is being able to minimize into Applications.
To be sure, this is the operating system we should have gotten when Leopard was released, and paying an extra $29 on top of the $129 for leopard is a little sketch. But it's been almost 2 years, so the money doesn't hurt so much.
Now as soon as Growl is updated to work with Snow Leopard, I'll be content. 😀