I have one off-brand (HK) - albeit maide in China (or HK ?) it is well made. I found it useful with longer lenses, use it routinely with my 90mm Elmarit on my M6. Makes focusing (and also composing) easier and more precise. Fiddling ? probably, if you switch lenses often on the go, but I do not and used to switch lenses for a long-term sessions duration.
The magnifier can also be used for 50mm lenses on x0.72 bodies. I used to leave the magnifier attached all the time shooting with either 50mm or 90mm lenses until recently, however for an obstacles wearer like me 50mm frame lines are pushed too far towards the edges making them hard to see, so I abandoned my habot of leaving it on the camera all the time, but only for 90mm lens.
Alex