I never went in expecting the prequels to be better or worse as the OT. But knowing what Star Wars was about, and viewing them as impartially as I could, I didn't feel (at least after the first 2) that they were of good substance, I supposed I am quilty of being biased though, at least of thinking I would enjoy them. I'll admit both 1 & 2 had some good moments here and there, but based on judging the first two on just being movies (Star Wars or not), I didn't come away feeling they were all that great.
Episode 3 really had a chance to be even better than it was, I just felt some moments here and there really took away from it being a great film, as it stands to me, it's an entertaining film, but with some tweeking, could have been so much better...
Thinking about things, I almost wish Lucas had only made episode 3, since it probably could have stood by itself (with a few changes) without the first two drawn out air-filled backstories.