I switched to Fritz Blue Box recently from Red Sea Pro, mixes better than any salt I've ever used and doesn't mess with my Alk and Cal levels like Red Sea pro or Reef Crystals.
I've only been hearing about Syn-biotic salt on R2R a ton recently, so no experience there. Tropic Marin is probably the company I trust most to get supplements and additives right (I dose All For Reef in my tank), so I can imagine they can back up their claims that Syn-biotic helps with fish stress and starting the tank. Then again, seems like the lot of us have never had any problems introducing cycling tanks and introducing new fish on the salts we've been using for years!
I am curious about Syn-biotic's benefits on nitrates and phosphates however.