I never really used it.
I mean, think about ALL the crap decks that were made. We focus on the upper echelon decks that wrung every last drop from these poor old dictation tapes these days. Stuff you only saw in magazines. But a crap deck with "Dolby" engaged, making a tape played back on an actually calibrated deck, is going to sound worse than two decks with no EQ curve added/subtracted in the chain.
So, I never used it. Pre-recorded tapes with Dolby generally were fine, again, on a properly calibrated deck. High-quality calibrated decks were out of my price range when new. Eventually I got high-quality non-calibrated decks, and finally, high quality calibrated ones.
I still don't bother with Dolby noise reduction...