When i decided i wanted to learn how to use photoshop, i went with CS3 because of small file size. Over time, i also got used to that version so i probably wont be downloading any other version even if i needed to. My ancient computer only has only 80gb hard disk capacity so i cant really afford to load it with huge programs.
But yeah, at the moment, i dont use any other version because im used to this one, and because this one has smaller file size.