We went to Nachi Cocum a few years ago and it was very nice. However, once you add in the R/T taxi fares with the admission costs it does get expensive. (The samething applies to most beach resorts Cozumel though.)
We did enjoy our stay. The food was fine, but not great (we get much better here in Houston). The beach was very nice, and the facilites were clean. We would probably not go again (and did not during our last visit to Cozumel), but only because we are becoming less of "beach people."
If you are looking for a nice beach day without huge crowds, it is worth considering.