CS3 is great so far for me. I've been using PSbeta for months now, so I was already used to the UI and speed advantage, but having a polished app is only better. I spend most of my time in PS, and it's really the most improved app I've used. I literally use some of the new smartfilters and black & white adjustment layers daily, I'd definitely miss these features if I went back to CS2. I use dreamweaver once a week or so, and it's definitely better (and a ton faster on intel) than MX, which is the last version I had, but it's not a huge upgrade in features that i use. Indesign and illustrator are both improved, but not incredibly. I don't use them nearly as much as PS, so I haven't noticed much difference outside of speed and look.
I didn't have any trouble installing, just ran the uninstaller included when i installed PSbeta and I was set. No problems whatsoever with file compatibility or stability; I never even got PSbeta to stop responding.
Truthfully I only really needed PS CS3, I could live with CS2 for everything else, but work covered most of the cost and it didn't cost me much more at all to get design premium. And boy oh boy is it faster, I can have lightroom on one monitor, PS on the other, and still preview stuff in acrobat or put it together in indesign with 1.5gb ram. Rosetta would not have liked that situation.