I am not a professional repairer and I have and use one. It gives me objective data, instead of subjective guesses. It can not only tell me that something is off, it can also answer the question of how far. Knowing that, I can adjust accordingly, often without the need for—or cost of—a professional repairer.

John, I have one, and on those occassions when I've had shutters tested professionally (usually after a check-up or repair) I've retested them with my humble little Calumet instrument and it was usually pretty darned close. For my purposes I trust mine.