I have been using photoshop since version 3.0 about10 years.
The first significant milestone in my opinion was back in ver 7 with the healing brush.
There are of course too many others to mention here, but that one sticks out for some reason.

With the advent of CS3 I can tell you, there are way too many improvements and feature enhancements
to pass up. If you are a PS user, start saving your pennies to purchase this version. It is without doubt
the most super improved, feature rich, and intuitive version Adobe has ever made to this industry standard
graphics program. For those who already are avid PS users, I suggest join NAPP (National Association of Photoshop Professionals)
http://www.photoshopuser.com/