I have stayed there three times. It is a pretty good choice. It is not really fancy, but nicer than many other hotels on the island. I understand it got some freshening up and upgrades with the post-hurricane repairs. The Fiesta used to be pretty much a divers-only hotel (wetsuits hanging from every balcony), but with the advent of all-inlcusive resorts on the island and the addition of the all-inclusive option at the Fiesta, there are now two different types of guests: divers and the all-inclusive party crowd. The party crowd hangs around the pool all day drinking and the resort caters to them with loud, obnoxious music and activities. As a result, the rooms in the tower can be a little noisy during the day. Thankfully, the poolside festivities do not go on into the evening. The "tropical casitas" behind the tower are pretty nice, and much more quiet, particularly if you want an afternoon nap. If it's in your budget, consider the casitas. They are usually not too much more than the deluxe rooms.
I am not a big fan of all-inclusives. We like to go to town for dinner. But the last two times we went there we took the all-inclusive option because it was just so darn cheap. The cost differential was equivalent to the cost of breakfast or a few beers, so we took the AI option and still went to town for dinner. Then if we didn't feel like going to town (say, after a night dive), we had the option to eat at the hotel. The food is decent, but not great. Breakfast is probably the best meal. Lots of fruit and one of my favorite Mexican breakfast dishes, chilaquiles.
I like Dive House, the dive operation at the Fiesta, which also has a location in the plaza downtown. They have bigger boats, so you have some room and don't get tossed around as badly if there are some waves, but they have been keeping the number of divers on each boat down, so you shouldn't be crowded. They use an 80 ft. profile for the deep dives and don't usually go to Punta Sur or Columbia, at least partly because of mixed experience and skill levels of divers. That's OK for me, because I am usually with a group from the dive shop. Dive House runs a very good, safe operation.
Taxi rides into town are readily available and pretty cheap. Car rentals are pretty expensive and it's hard to find a place to park downtown. I have rented cars a couple of times for the week and have now decided that I really only need to rent a car for a day or two to explore the island.
I will definitely go back to the Fiesta, although I liked it better before the party crowd started going.