We did it back in May '03. It was a blast! Whether it's worth the cost or not is a matter of opinion. Once you're there, the split everyone up into two groups: one plays with the critters while the other chills on the beach (they have chairs and umbrellas and such). Then you'll switch after lunch. With the animals, they had three separate stations: dolphins, stingrays, and hermit crabs - don't know if that will have changed by now or not.
With the dolphins, you wear a life vest and float in a deep tank with the dolphins. There were two separate halves when we were there, with a new mother and baby on one side. You get to swim around, pet the dolphin, kiss the dolphin, but no hanging on and riding. It was still very cool.
The stingrays are in a much smaller wading pool. You get to pet them and feed them, which is wild. They suck stuff up like a vacuum from your hand. You race the hermit crabs, which is somewhat amusing.
We thought it was nice day, but it is expensive, so I guess that may be a factor for some. They also have pictures for sail of you kissing the dolphins and such.