Today I briefly tried Adobe Photoshop CS4, and from what I could tell it appears to be a nice improvement over CS3. Since I'm now an almost entirely linux user, I'm not using Adobe products very much, but I have for years used Photoshop and Illustrator on Macintosh computers.
I've come to really like the new CS series, and I find both Photoshop and Illustrator CS3 to be big improvements on prior versions. But perhaps the most startling difference was with Adobe's InDesign product. When one of my customers switched from regular CS to CS2, we all realized to our disappointment that the two versions were not compatible! That was very disappointing, and I hope Adobe will not make the same mistake going from CS3 to CS4, for InDesign or any of their other programs.
Though as I said, I'm now a linux user, so I'll be using the GIMP and Inkscape.