I have been using
Photoshop since the version 3.0.5, so far skipped only the version 8
(CS) that had a major security problem. I'm currently using the version
9 (CS2) and will skip the 10 (CS3) because it does not offer anything
particular that I would need.
The major selling
method of Adobe seems to be that they tie the Adobe Camera RAW pluging
to the latest Photoshop version. So, in case you buy a new camera you'll
also have to buy the Photoshop upgrade. Clever but not decent. But I'm
still happy with my Canon D60, oooh, as many as 5 years old dSLR!