We've gotten two - one on each of our last two trips. One was a pink unicorn (DD got to pick herself) and one was a green dragon (2 DD's and one DS chose for the entire family -- all agreed a dragon would be fun). She's wonderful to watch no matter what, even if you don't get one to keep.
Both times we got there a little bit early (probably 10 - 15 minutes, at a time that wasn't too busy). We were the first ones there so the kids were right up front. I'd stand to the side a bit so that I wouldn't block other children.
We encouraged our kids to be polite (no pushing or begging) and to say hi when she came out (I think they said hi in Japanese).
We had an idea picked out in case they were chosen. They knew in advance not everyone gets one and that they would have to share if they got one. They were fine with that.
Ours lasted a while. I think we tasted it once, but I don't remember much what it tasted like.
We'll definitely look to see her next trip. She is mesmerizing!