I will put my 2 cents in. I previously used TM Pro for 2+ years in all three of my tanks, but the last couple buckets (in early 2010) were solid bricks. Nothing like having to chisel your salt mix right out of the gate. So, I switched to Reef Crytsals with no issues.
Recently, a large online vendor was offering free shipping and a 10% discount on orders above $150 for Black Friday. I figured I would give TM Pro another try, and placed an order for two buckets. Keep in mind, this was the weekend following Thanksgiving. The salt was on backorder, but only for a few days (at first). Twice afterward it was delayed, until it was a 2+ month backorder and I was in danger of running out of salt.
I called the vendor who said that Tropic Marin was having issues getting them salt. So, I changed my order and got 3 of the 200 gallon boxes of Reef Crystals for less than what 2 buckets of TM Pro would have been. And after reading these additional concerns, I will stay clear of TM Pro.