I got one from messrs Rotagrip and used it to tap 192 M4 holes in silver steel with a form tap. This was for the roller slides for my bench draws. I think it was worth it in the value of the taps I might have broken doing the job by hand
. Since then, it's been used to tap about a hundred M6 holes in aluminium and a few other jobs.
Basically, they're not a lot of use most of the time, but as soon as you get a project that needs half a dozen or more threaded holes on a flat surface, they are a very useful tool to have. Had I known that 20 years ago, I'd have only bought spiral point/flute and form taps, since they can be a bit fraught with hand taps!