The Great Barrier Reef is amazing in its own right but I dont think there can be many better ways to see it than with Mike Ball Dive Expeditions on Spoilsport.
Although the boat and the food were fabulous, the staff are the true stars of the show, entirely professional, very knowledgeable and lots of fun :) They brief you very well on all of the dive sites and their main features, sometimes it seems they even map the fish as they're often right where they say they are! Its then down to the individual diver as to whether they go with a guide or not. We did a mixture of both and had a fab time. They are pretty excellent at spotting things though.
We did the 3 day Cod Hole trip but will definitely be back one day to do the Coral Sea. Dive conditions are great, "bad" visibility is 20-25m so pretty stellar by the standards of many other countries (Especially the UK!) and there is plenty for the macro lovers too, nudibranchs galore! The reef out at the ribbon reefs is also in much better condition than that closer to the mainland.
If you have time and can be flexible, its WELL worth waiting for a last minute standby cabin. You can get some excellent deals as we did if you wait until 3 days before the trip departs. Equally though, its going to be worth whatever you pay. There is also something for any diver regardless of experience, I did my 50th dive on board and my partner his 40th but there were fresh OW divers on board and also some who hadnt dove in a while and there is always the option to snorkel or pass on a dive if you dont fancy it or want to visit the bar instead.
Enough from me, GO and experience it yourself and you will soon rave too :)