My boyfriend and I rented Casa Margarita for 7 days in August. I have stayed in many different types of accommodations all over the island from all inclusives to cheap hotels, and in my opinion Residencias Reef is the best place to stay in Cozumel.
I would recommend a car, as the condos are several miles south of town. There aren't any restaurants or grocery stores around, but you are situated on the most beautiful stretch of sand beach on the island.
Casa Margarita is on the first floor of the building, facing the infinity pool and beyond that, the ocean. From looking at the map of the property, you might think it's set back too far, but when you are there, you have the complete experience of being right on the beach. It is perfect for scuba divers as you can walk straight to/from the dock, past the rinse tank, and right to your back patio. Truly, it doesn't get better than this in terms of ease of diving. It also helps that the condos are a 2 minute boat ride from some of the best dive sites in Cozumel (not the case of you stay in town).
As for the condo itself, you'll have all the comforts of home...actually, in my case, better than my city apartment back home! The bathroom is huge with a walk in shower and lots of counter space. You have a full kitchen with plenty of pots/pans/utensils. The condo is really large and I think you could easily fit a family of four.
Casa Margarita met all of my expectations and I would highly recommend it over any resort on the island!