I found them terrible films, virtually unwatchable, boring, consequence free and painful to the eyes.
And don't go complaining that it's just the hype that backlashed or some other flimsy excuse, the prequels were the first Star Wars films I watched as a kid so that kite isn't going to fly.
They're just so badly made once you look past the special effects. The characters are all boring robots or something, with a few minor exceptions, usually when someone acts completely out of character.
An example of how these films fail is how they "introduce" a major villain in the third episode, General Grievous, who could have been awesome, if you knew who he was or why he was doing what he was doing.
Apparently this got explained on a cartoon show for kids before the film came out, but COME ON, who the fuck writes his characters as if everybody saw the kid's show?
Another big, big problem, something that failed the entire three prequels for me, was:
1. Anakin Skywalker/Darth Vader was the most important guy in the prequels.
2. The character was exceptionally bland, and not because of the actor, but because of how he was written. In the first film he is just a boring kid, in the second and third film he looks and acts like a bad guy before he is supposed to be a bad guy. His character arc is nearly non-existent and makes no sense.
There are some good points in the film, like the effects (When nobody is sitting down on a couch to talk about the plot, obviously), and I liked the action scenes. Too bad you rarely know what is going on in those scenes, but whatever.
TL;DR:
The films are bad. Not because they couldn't live up to the original films, but because they where just poorly written and directed.
Hey, do you want to watch an old science fiction film were all the things that went wrong with the prequels has been done right? Go check out Things to Come, an adaptation of the H.G. Wells story (With screenplay by him), made in 1937.
Do it now, it's in the public domain so you know how to get it.