We did the Atlantis excursion back in July '03. The beach is great, the aquarium is awesome!
On the downside, you take a ferry across to Paradise Island, and have to walk across the island to the resort. It's a long walk and we had a very large group so we had to stop several times to make sure we were all together. The guide walks you through the resort and pool area, pointing out where everything is, and then bring you to the beach and then you are on your own.
My Parents went to the casino while we took the kids to the beach. We met up a little later and toured part of the aquarium on our own, then headed over for the included lunch.
We only saw part of the aquarium, (shark tank area), decided to spend more time at the beach. Would love to go back again and see the rest of the aquarium.