I don't use Nutri sea water. I guess for one reason someone buying that. Would be they don't have an Ro/DI to make salt water. Maybe another reason would be only adding a small amount like into a Nano . Buying already mixed Sea salt will get expensive very quickly. It might be good for help start a new tank. I think it's a good habit not to mix different salt and stick to one brand. But everybody needs are different. I always wonder why people would buy already mixed sea water. Especially paying $12 to $15 for 4.4 gals for that stuff.
Some people say they have mixed results, same with using dry rock. I just think that there are too many variables to pin point anything.
I totally agree with the cost, it's a tad expensive. I was interested in trying it behind the fact that it contains Contains 11,000,000+ live bacteria. I see the main problem with this is the cost factor.