My son loved it! And he was only 4. He was "just" tall enough to ride.
I might add that he is very timid about most rides...very scared of the Haunted Mansion, Splash MOuntain. He will not even think about Space Mountain.
Really, I think it is harder on adults than kids!
I would like to add-- I am not exactly sure how well my son could see the screen, because the screens seem to be at an adult eye level. But it must not have bothered him. He obviously could still see what was going on. Also, the shoulder restraints that come down hit him at about the middle (or maybe a little lower) of his chest. But, he seemed safe and content. It is his favorite ride.