I did this tour in 2006. It was pleasant but I wished it lasted longer. Almost half the tour time was eaten up in transportation to and from the place (which is not really an island, but rather a beach area of Cozumel that was cut off by water from the mainland after a hurricane). You get a nice beach, an open bar that is quite rustic (think beer and soda more than mixed drinks), and a buffet lunch that wasn't bad but then it wasn't great either. We were lucky to be the only ship in port so we had the whole place and just 50 or so passengers who had booked it.
It was enjoyable and I can recommend it. Just be aware it is isolated and pretty basic. But then, that is a good reason to go.