ive been. stayed on ambergris caye, which is a popular tourist destination. little island off the coast.
getting to the island we took a boat, crammed with locals and food. going back we took a tiny plane.
most people get around on the island by bike, golf cart, or boat. its pretty small, though i think there has been a lot of development since we went.
belize has 2nd longest barrier reef, so some great diving out there (first place i did ocean diving). it also makes teh beaches less beachy, no surf. lots of sea grass. swimming areas are usually just places they wiped out the grass.
aside from diving did a couple trips back on teh mainland. one to visit the casino. another to do a jungle/mayan tour. pretty fun cruising around on an old schoolbus drinking rum punch. the mayan stuff was cool, not as impressive as the ruins near cancun (ex. chichen itza).
pretty laid back place, i enjoyed it. drank a ton of belikin. no real nightlife where we stayed, that i know of. days were mostly wake up, dive, lounge in hammock reading and drinking beer. repeat. ideal vacation.
can share pics if you are curious.