Well, I've been using the PhotoShop CS3 beta since the day after it came out, and I have to say I am really pleased with it. I definitely can't go back to CS2 now, Adobe's strategy to make me upgrade sooner has worked!
I felt a noticeable improvement in performance with the new version, which was a pleasant surprise as I was expecting a new version would have more features and be slower to run, as is usual with upgrades. But startup time and general processing time is definitely better.
I was a little uncertain about the interface changes at first, mostly out of unfamiliarity, but now that I'm used to it, the collapsing palettes are awesome. My screen is so much cleaner! I work with two monitors, so in the past I always had my workspace set up to have my palettes on one screen and my work area on the other. Now I can pretty comfortably work with everything on one screen, thanks to the handy collapsible palettes. Sometimes I still use both, but it is definitely better use of the screen real estate.
The new curves editor is sweet - you can see each color channel's curve and edit them all from the initial screen, and the curves are laid over a histogram making it really easy to see what adjustments need to be made.
The smart object editing has improved too, opening and placing PhotoShop documents into others is convenient.
I haven't had the opportunity to use the "Device Central" but I plan to check it out since I might be doing some Blackberry development soon (oh joy). The new version of Bridge is better, they seem to have gotten rid of or moved that annoying Bridge Center dashboard (it never worked as well as it should have). The integration with Acrobat Connect looks promising.
One feature I am hesitant to try? VersionCue. After my woes with the last version (HOURS of lost time recovering/fixing corrupted projects, general frustration), I plan to let someone else figure out the issues first.
If you have not downloaded the Betat yet, I'd say it's definitely worth a try. I haven't had any problems with stability or bugs, but check the Beta forums to see if there are known issues with your setup.