My wife and I dove with Divers Down on in January last year. I thought that they had a good operation. We stayed at the Marriott on 7 mile beach and they have a boat that does pick-up and drop-offs there. The crew was friendly and punctual. We also did Stingray City with them, they picked us up at the hotel and drove is to the boat. I think that the largest group we had on the boat was eight divers and we normally had between four and six divers in the morning and eight in the afternoons. The morning boats also left 7 mile beach before the cruise ships started off-loading, which was nice.
My only complaints about them was that they did not always have a second crew member that stayed on the boat during the dive, although, I think that is a standard practice on Grand Cayman. Also, when we did the Stingray City dive they did not have enough weight for everyone, however, that was partially due to one person using 30lbs.
They offered two-tank morning dives and one-tank afternoon dives daily and had several boats available. The boats seemed to be in good condition, we dove primarily on the pontoon boat (Casseope I believe), and they had a good system set up.
The staff were all very nice and I spoke to Steve several times prior to the trip regarding class offerings. If you take a specialty class, I would recommend that you purchase the reading matieral before hand and go through the books prior to the trip.
Finally, I really recommend doing the Stingray City dive. It was certianly touristy, but it is a really neat experience. If you are looking for transportation around the island, look for the vans with a yellow placard in the window. They offer very reasonable rates up and down Seven Mile Beach and to Georgetown.
Enjoy your trip!
Ryan