It is used pretty commonly here in the Pacific Northwest. My brother was shown the difference made using the stuff by a mechanic. There was absolutely no salt buildup in the engine maintained with Salt Away, pretty significant buildup without it. The claim the mechanic made was that the encrusted engine was also regulary flushed with fresh water. I don't think the mechanic had any reason to recommend it except that the marine center he worked at sold it. (Practically all do here.)
In practice you flush the engine normally with fresh water and just before shutting down, switch to a Salt Away solution with the mixing device provided with the kit. This leaves a solution in the little places that don't drain in the engine. It is very easy to do. I use the stuff almost every rinse. After the engine is shut down the mixing device is connected to my trailer brake wash out and then the remainder is used to rinse the trailer. The way the device works is that the chemical is continually being diluted as water flows through it so the brakes and trailer are getting a less concentrated solution.
Is it worth it? Probably, but sometimes I wonder. My dive gear only gets a fresh water bath and there is no salt buildup on it either. But I can't look inside my engine without a LOT of extra work, so I take the belt and suspenders approach.