MacWorld puts that question to bed in this detailed report.

With CS5, Adobe shows once again that it keeps pace with, and even stays a step ahead, of our changing times. When thinking about upgrading to CS5, it’s reasonable to want to sprint, to take the shortest route to the fastest result at the lowest price. I wouldn’t blame you for thinking that way. But if you can, train for a marathon instead. Consider that the investment you make now will pay off in the future…