Many darkroom people use it in place of packaged hypo clear. Twenty grams per liter of working solution - pretty cheap at a dollar a pound. Prepackaged hypo clear has a buffer and perhaps something to give it a shelf life in the package. But if it s used one-shot, plain sulfite will suffice. I use three liters or more every time I make prints.
Sodium thiosulfate anhydrous of fixer fame is a well known darkroom material, too.
I'm planning to stop using rapid fixer for making fiber based prints. Mostly it s the smell I don't like, but I also have become a little mistrustful of the short fixing times claimed for it when used on paper at the higher film concentration. Some papers, such as Kodak Elite, are known to require a longer time, even in a so-called rapid fix. I know sooner or later I will suffer for relying on it, so I'm dropping back to the old tried and true fix. He also discusses F24, a non-hardening fix.