When travelling, we usually like to stay in more "local" places with their own kitchens, etc, but when we are in Akumal we have really enjoyed Akumal Beach Resort. As stated above, the beach is incredible, snorkelling is great, and the rooms all face the ocean and are a stone's throw from the water.
Akumal Beach Resort is not the same old, same old AI. Its Italian owned, and has a European flavor. Its relatively small and unassuming. The food is pretty good, but as you know, there are great restaurants nearby, as well as in Playa del Carmen. The convenience of a good breakfast buffet, bottled water, a good pool, and other amenities make it worthwhile.
If you want to stay at an AI that doesn't feel like the typical giant, never leave the grounds AI, ABR will make you happy. If you want luxurious lobby area, a choice of restaurants, super fancy pools with swim up bars, tons of amenities, elevators, late night disco parties, etc, then ABR isn't the ticket.