I think when Lucas got it wrong is when he tried to make the prequels more "kids friendly" and "funny". The original trilogy was not funny, it was a dark story (although that has also been changing over the years from release to release). I am not a hardcore fan so I think overall, aside from these funny moments and Hayden's terrible acting, the prequels work pretty well.
My favorite battle from all 6 movies is the Duel of Fates in Phantom Menace and one of my demo scenes for surround is the Pod Race.
Although I prefer the old trilogy, I'll probably end up owning the complete saga.
