My wife and I just returned from the Brac a couple of weeks ago. We stayed and dived with Divi. We saw the Reef Divers boat several times, and other than appearing to be a little crowded some days, I can't comment on their service.
If anyone is considering Dive Tiara, I highly recommend them. DMs Scott and Gary were very courteous and professional. They would load the gear onto the boat and hook up the BC/reg to a full tank before you boarded each morning. The boat was never too crowded (usually 10 - 12 divers) and there were no rigid return times. You could either follow the DM or dive on your own. Suiting up was really easy. You walk to the back of the boat, sit down on the deck with your feet resting on the platform, and Scott or Gary would bring your BC/tank to the back and assist you in strapping in on. After putting on mask and fins (still seated at the rear), you just stand up and step off the back. This was my first experience with this level of service, and we were quite spoiled by the end of the week. To top it off, they removed from the boat, cleaned, and stored your gear after each dive.
For anyone considering a stay the Divi Tiara, the rooms were spacious and clean, though a little dated. We ate most meals onsite and for the most part the food was very good and the staff very responsive.
I would opt for some dives at Little Cayman if Reef Divers offers it. The deep wall dives on Brac were almost as good as those on Little Cayman, but there was no comparison between the shallow dives. LC's shallow reefs were in much better condition, and you'll have a much better chance to see sharks on a LC wall dive. We dove 'Mixing Bowl' and 'Hole in the Wall'. Both dives were great and we saw sharks both times.
