I have been a steady user of Photoshop since my initial purchase several years ago and I currently have CS3. After reading several reviews of CS4, I am leary of upgrading from CS3 since it appears that the improvements are mainly organizational and convenience. Thus, I am hesitant to make the investment.
Am I missing something or am I incorrect in my assessment? Hopefully there will be someone out there with first hand experience.
Can't speak from experience but I have two thoughts:
(1) Can't you download a 30-day trial of CS4 to give it a test run?
(2) I think you need CS4 if you want to process EOS 5D MkII files. As usual, Adobe don't offer backwards support for cameras that came out after a new version of Photoshop. This would also presumably apply to any new DSLRs that appear this spring or this fall, since they have said that the 4.6 update for camera RAW was the last one supported by CS3. The 50D (and XS) is supported by CS3.
If it doesn't have anything that makes you say "I gotta have that!", than pass. Adobe offers the same upgrade price going back several versions, so it will cost you the same to upgrade from CS4 or CS3, and in fact it will be effectively cheaper if you skip CS4 and get CS5.
Treat it like the Star Trek Movies: Only the even ones are any good. 8^)