Used to use IO, since decades ago, when that was all that was available. Not certain why I switched... looking for something better, I suppose.
When BRS started doing their salt testing, I was using Red Sea. Red Sea left a tremendous amount of guck in my mixing barrel, so I watched BRS's testing results, and switched to HW Marine Mix. Shortly thereafter, it went out of stock. Not having what I need available is simply unacceptable... so I switched to TM Classic. Until it too went unavailable.
So... I'm evaluating my options...
From BRS's clear mixing tests, Tropic Marin, HW Reefer, and Regular IO are all, to my eye, equivalent.
From BRS's Calc/Alk/Mag testing, Tropic Marin Regular was 375/10/1230. TM Pro was 450/8.5/1380. Regular IO was 400/11/1350.
From a cost standpoint, IO is by far the least expensive. Chewy has it for $40.72 for the 200g box, delivered.
From an availability standpoint, IO is everywhere, and you get your pick of packaging.
Why was it most of us moved away from IO? Chasing something better? Not sure we ever found it.
Oh, Shedd's Aquarium uses IO, as does Georgia Aquarium, Bass Pro's aquarium... Works for them.
I went back to IO, and unless something dramatic changes, I'm done worrying about salt mix.