I've tried it before. Actually bought an entire bucket.
I think it was perfectly good salt. The most noticeable thing about it was that it mixed really, really well. Literally it would be mixed without any crystals left at the bottom of the bucket after your first stirring when you add it in.
The second thing I liked about it is that each bucket was consistent. I tested the bucket every four to six water changes and the parameters were always near identical.
The final thing I liked about it is the salt seemed to not leave a residue in the bucket, or my powerheads and heaters I use in my mixing buckets.
I'm now using coral pro from red sea. The only reason I switched is I wanted slightly higher parameters, and always like to try something different. Next bucket with probably be Kent Marine, then aquavitro salinity.
I'd like to test them all to be honest with you. I'm sure there will be people that'll snicker at fluval sea salt, but it's high quality salt.