Installing on both a MacBook Pro and a Mac Pro were two different stories. On the laptop, things were ducky. Everything worked, mail accounts were there, bookmarks, the usual.
On my desktop it was not so pretty. I kept getting messages that I needed to add a password for my MobileMe account, which I dutifully did. Problem was it kept asking. Then I saw some messages about the keychain. Whoops. Not a good message to see.
Well, the short version is that somehow my keychain was hosed. I tried to repair, but that didn't stop the problems. I finally threw away my keychain data and started again, re-entering my email passwords and a few other things. All is back to normal. I also saw a strange message about my iDisk, but I followed the directions and threw a rather large file away. It appeared to be a copy of my idisk for local use.
Other things pretty much work. My Sonos audio system is fine, Photoshop CS3 seems OK, and while it didn't get a heavy duty run through, it opens images, applies filters, runs actions, and saves properly.
Shut down is almost instantaneous, and I noticed I'd saved 14GB after the upgrade.
I'm seeing lots of little niceties. When I take a screen shot, it has a name that includes the date and time instead of 'picture 1'.
All in all, not too painful, but not completely trouble free. With keychain passwords gone I may wind up doing some extra typing for awhile on web sites where I have an account, but that's not too horrible.
Snow Leopard is clearly faster, trimmer, and a bit more fun to use. A good use of $30.00 and just a little bit more time than I planned in getting over a couple of glitches.