I've visited three times but not for a while. Everyone usually goes to Ambergris Cay and it's worth it, but I always also head up into the Cayo District in the mountains. It's spectacular in terms of the jungles and rivers. Placencia down south is okay but nothing exceptional. A lot of expats there and it can be fun if you're into meeting people and hanging out in the bars.
The beaches in Ambergris are not what you might normally expect in the tropics. They're rocky and not very white, and the sea can be rough. But the culture and laid back lifestyle are the main selling points. Caye Caulker (sp??) is worth a day visit.
It is hurricance season in Sept but that's when they have the best deals and the risk is worth it to avoid the crowds I think. The last time I was in Cayo I stayed at Black Rock Lodge. Absolutely amazing but not sure if it's still around.