I just had my first scuba experience on my honeymoon in Cancun during mid-May. My wife and I took a resort course and were given enough info to keep us alive. Then we had some pool time 2.5 hours later we were boarding the dive boat. The largest operation down there is AquaWorld..
http://www.aquaworld.com.mx/ . For the most part, the equipment provided was in very good shape. Everything was in great shape and functioned correctly. I did however have one small problem.
Upon getting to our destination dive over the Manchones Reef, about a mile offshore from Isla Mujeres, after donning all gear and getting into the water, my reg was not responding well to inhalation when I got about 10 ft under the surface. Of course, this being my first dive ever, it was a bit nerve wrecking. However, they do have a first rate selection of DM's that accompany you. Every few feet they make sure you're doing OK. Well when our DM asked me, I signaled that I was having problems with my reg. He immediately tested the backup octo and passed it to me underwater. Once I tried it, everything was great.
We continued our descent to 40ft and then had about 35 min of bottom time. Everything was fine after the reg incident. I guess something like that could have broken alot of people's spirit to continue the dive, but I was fine.
So, just from my 1st and only experience and having trouble with rental equipment, YES..they do have problems. My advice would be to test the equipment before getting into the water. They cater to so many people I really don't see how they have the time to do daily tunings of equipment. Either way, I'm here to tell about it and am pursuing a LDS to get my OWC. I got bit by the Scuba bug really hard.
Another LDS down there you might want to check into is the Cancun Dive Club. It's on the north end of the island right across the street from the MONSTROUS Mexican flag flying high over the city. You can't miss it. We have friends that used them and had nothing but raves from the quality of the uquipment to the professionalism and friendliness of the DMs.
Most of all.. Enjoy yourself.
Chris