Disclaimer: MGR Productions sent me their Bresson V1 zoomable viewfinder (1,1x - 1,6x). It goes for 84-90$. Apparently, it is now discontinued and has been superseded by: Adjustable viewfinder magnifier Type J and Y.

A zoomable magnifier is a clever solution to a perennial problem: the Leica M's non-TTL fixed OVF lacks both native diopter correction, and automatic tracking to a lens’s native FOV. After being asked by dozens of readers to check out MGR's zoomable Bresson finder-- reported by Leica Rumors in August of 2015 -- MGR sent me one for review. Many thanks, MGR.

I have used it daily (and in one commercial shoot) for four months, I would like to share the following pre-review observations:

there’s no more cost-effective quality magnifier out there, price regardless.
the Bresson packs the power of three magnifiers into one.
it is light
its operation is simple
if you wear glasses, it occludes frame lines wider than 75mm.
its large rear-lens reflects a lot of light.

Nevertheless, MGR’s Bresson is clever. In order for it to obstruct less of the viewfinder, its rear lens would have had to be much larger. In order for it to be more compact, it would be useful only for 135mm lenses. Overall, while elegant, and certainly useful, it competes essentially by merit of its price/performance ratio. And because of that, it is a truly interesting, if truly flawed product.