it is a good accessory, I use it on my leica IIIf because the image in the rangefinder is kind of dim, keep in mind though that it would probably not give you a good framing for your photos since it is not by Leica and who knows what camera body it was designed for.
These weren't made for composing your photos. Just for focusing. This particular model is reasonably accurate, although its narrow base limits its accuracy.
Still, it should keep you within the margin of error of most lenses, although possibly not the high-speed lenses (f/1.4 and faster).