We reluctantly booked Atlantis through Disney last year, based on significant enthusiasm from our youngest son. We wanted to remain open-minded going in, and I think we did.
The positives: Disney transportation was really easy and the drive there and back was interesting. Loved that we didn't have to worry about logistics. Our youngest can always venture off with our adult sons, so my husband and I spent most of the time at the beach. Some of the time, the beach areas were pleasant and relatively uncrowded. The water was beautiful and clean.
The negatives: The waterpark was crazy crowded, the lines were long, the food was not great. It was, of course, super expensive. My husband and I went on exactly one waterpark ride and the kids did maybe four in the time we had with the crazy long lines. It's very Vegas-like, and we don't love that (although our youngest was with us in Vegas for a family wedding once and he thinks it's fantastic, so this can be a positive or a negative, depending on your perspective.) Part of the time at the beach, our fellow beach goers really loved using profanity - and were super loud about it. But that's just the luck of the draw.
We definitely won't return. It was an expensive day, but I'm glad our youngest was able to experience it, given all his excitement since he saw it in the distance during our Dream cruise in 2011. And the weather was perfect. I like the Blue Lagoon suggestion that others posted. This year, we're getting off the ship again but booked the Blue Lagoon Dolphin swim. If we enjoy Blue Lagoon, we'll probably book the beach day there in the future.