Firstly it is FAST!!!! Well at least on my 2008 MacPro (16GB RAM). The way windows, scrolling and other general Finder behaviour operate is noticeably quicker.
No obvious problems with Photoshop CS3 or Lightroom 2.2.
Canon Pro9000 - OK
Epson V700 - OK
Canon FS4000 (via VueScan) - OK
Firmtek 4-bay enclosure (via Sertek PCI eSATA cards) - OK
Canon iPF5100 - No go. New drivers required. Full 64bit driver for *000 & *100 printers (and Photoshop Plug-in) due around Oct 9th it appears.
Needed to reselect my monitor profile.
I have a SuperDuper backup (which is why I was happy to install SL so soon) I can boot into for iPF5100 printing, or if anything else rears its ugly head, but it will be a hardship to do so, the improvement is so noticeable!
By the way I did a Customised (removed non-English languages, X-11 etc) standard Install over the top of 10.5.8. I've always done Archive&Install before but was curious to see see how the default Install worked. Rosetta was installed via Software Update after the OS install. I don't know how much is due to the smaller OS and how much was the removal of incompatible software (you're told some has been removed, but not what) but I recovered 38GB of space! Note that SL has changed how space is shown. The OS now uses 1000MB = 1GB, not 1024. Also the Mac default gamma has changed from 1.8 to 2.2. It's like loosing the dogcow!