BTL is the only service provider, and I have spent many hours cursing the computer screen trying to do uploads and getting halfway there and BTL takes a dump. But the internet access is acceptably slow. I was in Cancun using internet cafes there and had worse speed problems. Your connect speed is at whatever BTL decides it will be when you log in. Sometimes you fly like at home and sometimes you crawl, or it stalls - doing nothing. It just depends on what the internet fairy grants you that day.
The six month wait is everywhere, but you could get lucky and get the service hooked up right away. If internet access is already installed at where you are staying, you can go to BTL and have your account activated and probably be online in a couple of hours. With hurricane Keith hitting at the end of September, it added additional work to the already maxed out company. Some residents of Caye Caulker still don't have phone service. I've heard though that there are rentals available with internet access. There is an internet cafe on Caye Caulkeron Front Street.