Why, you're in luck!
viewtopic.php?f=15&t=21639&hilit=isla+mujeresOn my most recent trip in January 2010, I took in no less that four meals over 10 days at Asia Caribe in El Centro. It is run by a nice couple from Maine that lives on the island nine months a year. By all means, do try to convince the kitchen to fix you their off menu special of fresh fish cooked in their interpretation of tikin xic. Their coconut shrimp over a lime beurre blanc is very good, too.
Recently, all of the travel websites have been rating a place called Mango Cafe highly. After having eaten there, I strongly suspect that most of those online reviews are written by the owners. The food was disappointing and the service was horrible.
Although I haven't experienced it myself, the once delightful Olivia, seems to be getting a bit too big for its britches with all sorts of new house rules that have taken effect as it has become popular. Folks staying at my hotel on the last visit complained of being grossly overcharged there.
Make a point to visit La Colonia Gloria early one Sunday morning to partake of the local cochinita pibil and some fresh warm tortillas. I always buy a kilo, take it back to my hotel and share it with other guests who are interested. Be aware that most stalls that sell itb usually sell out by 9 am.
The all night Tacos Cachirul opens at 8 pm and is the best taqueria on the island. It is on the southern end of the island down near the Salinas Grande.
Board poster MST first introduced me to Isla Mujeres. She blogs about it frequently here:
http://sweettooth.typepad.com/mst/isla_mujeres_mexico/