Went there for a week with the family (wife, daughter and the in-laws); We stayed on the Ambergris Cay - 15 min flight from the main land (NE);
In short, the visit was very enjoyable and relaxing; The people are extremely friendly and seem very happy in general. It's definitely a second-world country (not quite third-world) with ramshackle housing here and there once you step out of the resort areas, but better than Honduras or Jamaica (from what I hear).
The marine life was very rich (lots of corals, sponges, fish, rays, sharks) - snorkeling was terrific, even our 5 year old enjoyed the snorkeling because the sea was very calm as the 186 mile barrier reef protects the island's windward shores.
One day we purchased an adventure trip; after two snorkeling stops and picking up some fish and big conch shells the guides made a BBQ picnic for us on the beach featuring the fish they just speared, the conch and lobster tails - I don't even know how to tell you how amazing that food tasted there on the sunny beach under a palm tree looking at the turquoise sea and a sandy beach...
Cracked open and drank some fresh coconut milk, and tasted roasted coconut - local delicacy;
All and all not a cheap vacation but all had a terrific time and came home relaxed; Now back to the 'salt mines'...
Hope you get to venture out there - it's both beautiful and peaceful;