I have tried to drink the IO Kool-aid a couple of times over the years and a few other brands along the way.
So why not IO for me?
One, it doesn't mix up to the parameters I want to keep in my tank. I know that I could "adjust" it to meet those parameters, but I don't see the logic in it unless no other product would meet my needs without those adjustments.
Two, as many have mentioned, when I tried to use IO (and RC) I would get that brown sludge in my mixing barrel, and while benign, I just didn't want to deal with cleaning it out that often.
Full disclosure, I've also tried these others which ultimately came down to parameter levels that my tank did the best with balanced with the overall cost: Fritz, Oceanic, Red Sea all didn't fit what I had in my tank.
What do I use? I use Coralife salt. It mixes Alk ~8-9dkh and the Calc and Mag seem to do fine for the corals in my tank.
I'm in the camp of..find what works for you and try to stick with it as long as you can unless something in the tank starts to decline due to some change in the mix or you introduce corals that really need different parameters to do well.
Happy Reefing!