We loved Aulani and the kids loved Aunties. We ended up taking them there 3 times at least. Twice for the two activities we pre-signed up for, another time while we were at the spa, and I think my son dropped in one more time. We were there the week after Thanksgiving, and although it was not a holiday week the resort seemed close to full. I was up early in the mornings and only once saw one mom waiting outside before opening time. It didn't make sense to wait early because they send everyone to the back door for open play and then you have to come around to the front to check in for drop off. But I guess if there was a long line there we would have been in it. We signed them up for meals when we dropped off. There were a few days when mid-day a sign said it was full for drop offs.
We did try once to sign up for another planned activity and it was full. Otherwise the sign up process was pretty similar to what's described above.
One thing I'll say about Annies that slightly bugged us: when the kids were there for free play they got a LOT of screen time. Both loved the video game tables and my daughter always ended up in the movie room. I get it, Disney is a media company. When we're on vacation we try to limit the amount of time they sit in front of screens, and most child care providers we use don't have any screen time except the occasional rainy day movie. So if that's something that bothers you, beware. For us it just meant it was really, really hard to pull the kids out of there because they were glued to screens.