We went to Belize, October 2005. We had a great time! We stayed at the Inn at Robert's Grove near Placencia.
As for the diving it was pretty good. You had to travel at least 45 minutes, if not more, to your dive destination. We would do a one tank dive in the morning and then a surface interval on one of the outer cayes. Silk Caye was beautiful (but the furthest out). After about a two hour surface interval and lunch (awesome barbeque that the boat crew cooked), we would do the afternoon dive. One thing we encountered in October was that the season was slow for that part of the country. The dive shop at the resort required at least four people to go out. If they couldn't take us out, we were farmed out to other dive operations in the town. But the other dive operations equipment and boats were not as nice. We couldn't dive the second half of our trip - hurricane Wilma was churning up the waters. However, the resort booked us a interesting Mayan tour. Nobody else from the resort was going, so it was just 2 of us and the tour guide.
I would definately go back, but chose better time of year for that region.
As for the other parts of Belize, I would like to try Turneffe Island and stay at the Turneffe Lodge. It doesn't look like there's much to do but dive there. That's great for me and the diving looks close. I would also like to stay on Ambergis Caye as well, but I have not researched where. If any other scubaboard members have oppinions on either, I would be interested to hear them.
Keep us posted on what spot you choose. Happy vacation planning!