I can understand where the feeling of being underwhelmed to a certain degree by PSI-III comes from. I, personally, played PSIV first and was unable to get my hands on the predecessors for a couple of years. In that time, I had read text walkthroughs of all 3 games and had visualized them in my mind with PSIV-esque graphics. Sure, it makes no sense technically, but at the time I was 12 and had only owned a Genesis, so don't give me too much crap ;p
However, despite my initial reaction to the more simplistic graphics, scripts and music, I did end up enjoying the older games just as much. PSIV is still my favorite of the series, but PSI and II are just as great. PSIII, on the other hand, I felt was a great idea with poor execution. Everything about it just felt rushed.
-Corey