I’m admittedly a late one to getting the most recent software, but today I finished upgrading all my Adobe software to the latest version (CS4), and have been pretty pleased with the results so far.
It seemed to me like CS3 was still a little clunky with leftover pieces from old Macromedia software and ran a bit slow on Vista; not nearly as bad a CS2 however. The newest versions run noticeably faster on my machine and have a number of neat new innate features that make each application more compatible with its associates.
I’d recommend upgrading to the new version. While I hate the fact that the new model for software is to force an upgrade every year, I agree that this upgrade was worth it. Check it out for yourself by going to Adobe.com and downloading the free trial, or click the link on the bottom right of my blog- I believe you get a discount.