I had one for approximately 5 years in brackish and it seemed to do well, until it escaped and never recovered from injuries sustained during the attempt. However, I have one now in marine conditions and appears much more energetic and robust than the one kept in brackish water ever did. Everything I ever read on this species indicates marine conditions as best long term like badguitarist stated earlier.
About the brackish fish in SW, I would think some species are not better off in pure SW longterm...such as mudskippers, Anableps, etc