FWIW I'll weigh in with my experience today.
I upgraded my MacPro and Air last night to SL. Did so for one reason, to test out Exchange hoping it would reduce my trips to Fusion/XP/Outlook. What I wound up with was no printers and the net dropping out every 10-20 minutes. Exchange worked though. I spent the entire day trying to get back to where I was last night.
My default printer is a < 1yr old Canon MX700 (networked), bought it from Apple, fully supported under SL, 10.6 drivers, etc. It flat-out disappeared from SL, worked fine with Fusion/XP. A new Brother laser I forgot I had showed up just fine, except I couldn't print to it. I know I wasn't alone with this problem.
The net-dropping started right away, had to power-down every 10-20 minutes (I was to spend the day tweaking a new ecomm site, that went well). Blogged around to find the New Location fix, things got better a few hours after that. Fired up a couple of Adobes, no major problems but no major advantages either.
Bottom line bitch is my issues were all-Apple. Apple made a hard-partner push with Canon (I got two), probably putting many thousands of these printers in the market in the last year or two. You would think they would want them to work right out of the SL box. Same with Airport Ex, where are they going to point to there, my Comcast modem?
These were really basic issues, the forums and blogs are filling up with them. My issues were not some five-year-old driver problem, outdated hardware, etc. This stuff is all less than a year old, totally up to date and has the best available software . . . and it takes two hours to install a printer under SL.
I have a 9:15 apt tomorrow at the Genius Bar to discuss how to downgrade.