Snow Leopard seems good speed-wise but I get the niggling feeling something, somewhere, isn't quite right with my iMac since upgrading. I've had a hang on shutdown a couple of times, and occasionally on start up it'll load the desktop but won't react to clicking on applications in the dock. Never had any problems with Leopard.
And then there's the straight bug - I have a Nikon camera which used to be recognised by iPhoto without a problem in Leopard and now isn't: it sometimes appears without any of the photos, sometimes doesn't at all, and I'm having to do a laborious file import round trip with a proprietary Nikon bridge utility. Is there such a thing as a manual driver refresh with OS-X?
I have Macbook Pros at work and home, they have no problem whatsoever (haven't tried the camera on those yet though).
Cheers, P.