My Wife and I used Manta Divers Last October (Prior to the Hurricane that trashed Cancun) and were generally happy with the operation and service, however, there were a couple of somewhat negative things.
We made reservations on their website for a package deal that included 3 days of 2 tank dives. Two day's of 2 tank dives at the Reefs around Cancun, and a 2 tank dive at Cozumel. Upon arriving at there store the owner tried to convince us that diving the Cenotes is much better than the Cozumel dive and we should consider changing our plan and Dive he Cenotes rather than Cozumel. As we have only been diving for a couple of years we thought the Cenotes would be a little too adventurous for us, although we said we would consider changing.
The first reef dive was just okay...! Our dive master didn’t do a safety briefing before the dive and once on the bottom, didn't seem all that interested in pointing out the reef life and things of interest.
The second day of diving was much better! Our Dive Master was a young local guy (Michele) who obviously took pride in his Job! He was very professional and once on the Bottom went out of his way to point out a huge variety of Fish and creatures in and around the Reef!
After the second day of Diving the Reef we plucked up the courage over a couple of Margaritas and decided to go ahead and dive the Cenotes. We figured that for the Cozumel trip Manta had to sub-contract to another Dive store on Cozumel and that it cost them more than the Cenotes dive, although that was just our theory and is not based on actual fact!
For the Cenotes dive, our German Dive Master (Life) is one of the most experienced Cave divers in the area. He did a good job briefing us on the Cenotes during the one Hour Van ride to Choc mool. The Cenotes dive was a great experience and one that we will definitely do again in the future!
In summary; we generally had a good experience with Manta Divers and considering some of the larger Dive operations in the area we would probably use them again.
Hope this helps!
"Better on the Bottom than on the Surface"