There are some interesting facts about marine aquarium salt. For instance, all salts (with the exception of the red sea stuff) sold in the U.S. are mixed at the same plant. Yes, that's all of them.
Anyhow, a study was done by Eric Borneman recently. Preliminary results show Reef Crystals and the Red Sea stuff to have ranked at the top, with Instant Ocean being the absolute bottom competitor.
Good was defined as better coral growth, stronger coral skeletons, etc. Bad was less growth, weaker skeletons, higher algae growth.
I personally have used Reef Crystals for a long time and like it very much. I am seriously thinking, however, of checking into the new Red Sea stuff. I have heard some REALLY good stuff about it from some folks that really know there stuff.