I realized it on the way home from the theater after seeing Episode I (didn't see it on opening night, but within a week or so of it coming out, iirc).
Of course I "hoped" against hope that the other movies would be better and as my good friend said "I'm hoping that when viewed in the context of the Prequel trilogy, Episode I won't suck, so I'm reserving judgment."
Episode II was better, but did have a lot of crap in it (which I was forced to admit once I'd seen the IMAX cut which was far superior). Episode III had those awful moments that ruined an otherwise diamond in the rough.
As a trilogy, I realized the Prequels would suck once I finished watching Episode III, there was just no way it could hold together as a series to stand up to anything approaching the OT.
I don't hate the Prequels per se, but I was defiantly let down by them, and not just because my expectations were so high. The continuity lapses with the OT were just inexcusable.
Mainly I agree with the claim that the characters in the OT were more likable, and their onscreen chemistry was better. The story was more upbeat (generally) and hence lent itself to that adventure theme that reminded you of having fun at the cinema. I'm not saying a dark, political feel was wrong for the Prequels, but that "deadly serious" atmosphere mixed with random cartoonish "humor" just didn't work well. The characters were too stiff and there were too many of them to keep us caring about any of them. Too often Lucas dialed down the suspense to give us self-indulgent moments of ships landing and goofy aliens doing slapstick. It's like he just wanted to play in the world and remind us what an artistic genius he is, rather than tell the story and get us to identify with the characters and their struggle.
I think the Star Wars name works against the Prequels. Had they just been a set of sci fi films set in a different universe, I don't think they would have been so harshly criticized, but the catch 22 is that I don't think they would have made as much money, since so many people went to see them out of obligation as fans or in expectation of seeing something approaching the "magic" of the original trilogy.
FYI: I avoided spoilers like the plague for all three prequels. I actually wonder though, if people who "spoiled" themselves silly for the movie beforehand were more disappointed or less than those who avoided them?