Apple is having a very hard time getting rid of the really big 10.6 bugs (emphases mine) ...
One minor annoyance: I noticed on both computers that my display's custom ColorSync profile setting had been forgotten. I had to go into System Preferences - Displays - Color and select the correct profile.
Please see also the report concerning the three Snow Leopard bugs originally identified and reported in TidBITS. Two of them are fixed by this release (great, that only took 7 months), but one, a serious bug where attempting to Finder-copy a file bundle via File Sharing, is not fixed. http://db.tidbits.com/article/11123"
The Tidbits article link reports fixes to an Apple events bug and a Preview bug.
I still can' recommend upgrading to 10.6 from a happy 10.5 machine. At this rate, it won't be respectable before 10.6.5. Apple is struggling to keep all their balls in the air.
There's been no mention of a fix for the USB/firewire external drive system lockup bug.