OK, here's my observations:
1) Whenever I do a system upgrade, I always test it out first on a separate partition, so if it goes south or has problems I need time to figure out, I can reboot into my old system and continue to work. I'm still using Leopard on a day to day basis, until I get Snow Leopard tested more thoroughly.
2) Everything seems noticeably faster: boots up a few seconds faster, applications launch faster (Photoshop CS4 is unbelievably fast), windows in the Finder display long lists faster. I like that!
3) Haven't personally had any crashes with the Design software I use: Quark, CS4 Suite (Photoshop, Illustrator, Dreamweaver, Flash InDesign), Suitcase, FontExplorer X Pro. But here's a link that has a lot of people complaining about CS4 Suite crashing: (also see #5 below) http://blogs.adobe.com/jnack/2009/09...ith_ps_sl.html
4) Snow Leopard doesn't support AppleTalk, so that potentially breaks a lot of older laser printers. I spent about an hour trying to connect to my old LaserJet 5MP, but no luck. There is an update for HP printers that is supposed to fix the problem, but it didn't work for me.
5) There are apparently problems with Abobe and other applications opening from/saving to servers with the SMB protocol. That wasn't addressed in the 10.6.1 update, either. http://kb2.adobe.com/cps/506/cpsid_50654.html
Not a problem for me, as the servers I administer are AFP, Mac only. But it's a red flag to me, because Leopard had problems between server & client that went unaddressed for over a year! I will be testing in this area a LOT more before I upgrade any clients to Snow Leopard.
6) Here's the most comprehensive site I've seen for compatibility info: http://snowleopard.wikidot.com/
That's it for now. My bed is calling me...