I recently went with Deep Sea Divers Den as drbill did on the OceanQuest and had a great time. It would have been nice to head up to the Coral Sea, but my schedule had to be around work and it was either spend half a week in Cairns waiting for a sailing up north, or spend that time out on the ocean. And I was already "stretching" my business trip from 4 days to 13, so I couldn't easily stay another week as well. So I chose this options since you can go out on any day of the week and come back on any day as well. Even if you do a different live aboard, it might be a nice way to fill in any extra days before or after your main trip since the cost is less than a hotel in Cairns and paying for day trips.
The full days your on the OceanQuest is a 5 dive schedule, 6:30, 9:00, 11:00, 3:30, and 7:00 (night dive). On days heading out you get in 4 dives, and days returning 3, so in 5 days I did 22 dives total. And no time wasted moving from site to site, though we usually moved a couple times a day, they do it during breaks between dives and the moves are pretty short. The reef may not be as colorful or as pristine as up north, but it was plenty nice and all the dives were easy and relaxing. Price was low, food was good and plentiful, crew was friendly and helpful, boat was spacious, and I even had cellular service to check in with the family since they didn't get to go. There were a lot of younger divers, and being an older guy I was a bit worried at first I'd be out of place, but I met quite a few new dive buddies and had a great time diving with folks from all over the world and finally the last couple days with a great "Aussia bloke" from Perth. I've posted a few photos up at:
Australia 2009