We did it when we were there the first weekend of November last year, which was a somewhat busy weekend because of the earlier hurricane and the addition of the Halloween nights. We had no wait at all and we went in the middle of the day, there was no one else in there.
Overall, I'm very happy with it. I'm a little annoyed about a few minor things, the worker who did ours was really rushing (even though no one was waiting) and didn't explain things well to the kids (ages 8 and 13) so a lot of our scenes are just really off, like the frog jumps out and then it's a good 10 seconds later before the kids jump in surprise.
I wish we would have been allowed to review all the scenes before purchasing, we would have redone it. They will only let you see a few scenes, which they choose. You don't get to see the rest until you go home and watch the DVD.
You probably already know this but don't wear any green. I was wearing my slytherin robe and when I turn around my back disappears in the scene, it's kinda funny looking.