I'd second (third? fourth?) the issues about incompatible apps that more than one person in this thread has raised.
After an annoying hang in my first attempt at installing on a newly erased HDD, I used the original reinstall disks from my 2008 MacPro to erase the boot drive and reinstate a clean 10.5.1. Then added 10.6 on top of that with no other offending apps on board.
It works pretty much perfectly as I slowly reinstall the other apps originally present. Even Mozy managed to figure out that I had manually dragged 42gb of iTunes files from a SuperDuper backup and actually opted out of reuploading the whole library. It's been a great excuse to clean out the remains of other apps that proved useless and that AppZapper failed to catch, as well.
Sure, there have been some glitches. Growl failed to install properly (see now-current disclaimer on their web site to wait for a new version), an older game (OSX Skyfighters, from the late Don Hill) hasn't made the transition, and over two days there has been one Firefox hang that required a force quit. But that honestly was probably as much operator stupidity as anything else: I'm still as impatient as I used to be with slow web sites.
I havent pushed that hard yet with other apps like an earlier Photoshop Elements: installing older Adobe software is just asking for hangs and crashes.
You could argue that both the Apple developers and the outside people had enough time to sort this out before the release, but perhaps responsibility could be apportioned to both sides. Its clearly going to take time to sort out what really works and what doesnt.
Overall, though, a speed increase in the system is easily noticeable to me and my best guess is that this is not just reformatting and reinstalling the OS
