I haven't been, but I'm researching Belize and there are so many different areas there that you could easily spend two weeks. My perfect two in March would start in the south for four days with a whale shark trip; move out to the atolls for another 5-6 and then spend 4-5 days around Ambergris Caye. Now why would I go there? It's relatively inexpensive compared to the rest of the Caribbean, the diving is varied, and there is plenty to do if I wanted to take a day off and do something else, like visiting ruins, taking a jungle hike or river tubing.
Let us know what you decide!
Gwen