I have also used many brands, but I Have been able to tell the difference. Through testing many of us have been able to find inconsistnacies in salt mixes, and it kind of makes some of us a bit angry that they can't give us a consistant product.
Without the proper magnesium your alkalinity and calcium may fall out of solution, which means you have to buy mag to dose with to keep it up, this also means a new test kit. If your alkaliniy is low in a salt brand then you have to get a buffer to keep that up and a test kit. Same goes with calcium. If your salt mix can not keep these three at least near what they should be then you can have trouble growing coralline algea, it may affect your corals ability to grow.
If you have a great brand of salt the first time doesn't mean it will be the same the second time you use it.
I personally have used a killer bucket of Instant Ocean, one of the leading favorite salts for many.
Point is when using a new bucket or bag of salt, no matter who makes it, test it. If it mixes cloudy and stays cloudy even to the point of foaming on the sides of the container then don't use it.
And of course buyer beware is in affect.
Right now I'm using Oceanic and it is working for me.
Thomas