I've been using it for close to two years now. My motivation for using it in the first place was consistent parameters. Using bucket salt in the past always resulted in different parameters each time I'd mix up 10-15 gallons because of the stratification that can occur in buckets. I would roll the buckets on the floor, flip them over and over again, mix it back and forth between buckets, but I could never get a consistent mix.
ESV's system has completely eliminated any variance in parameters, every time I make salt, I know it will mix to 10 dkh, 450 ppm Ca and 1450 Mg. No mater what quantity I mix up. Other bonuses to their salt is it is the cleanest mixing salt I've ever used; there is absolutely no residue left over in the mixing barrel, and no precipitation. You can also use the salt almost immediately after mixing (not that I've ever needed to).
It is pricey, but I buy the 200 gallon kit for about $95 shipped to my door from marine depot, and that lasts a couple months for my system.