A new life for the beautiful old 645 lenses
I purchased the adapter to use my old Mamiya 645 lenses on Fujifilm GFX. The ability to give an extra life to the old medium format lenses, that have the beauty and the optical character, is such a big deal. The built and optical quality are excellent. When you add an extra F-stop to the performance of your existing lens is a significant reason to buy that speedbooster adapter.
I wish it supported AF, like a few other manufacturers have (esp. for Contax lenses, etc.), but I am not sure about the technical difficulties or feasibility issues in its production. But there is always a way to solve a problem. On a mirrorless system, it is not a huge problem, though, even if it lacks of the convenience of a straight AF. I find it sometimes even a better option rather than leaving the control to the camera's AF algorithm. But it isn't a good idea to adapt it on an SLR camera, even if one would have eagle eyes.
The price is not inexpensive, but if you have more than one Mamiya 645 lenses, you might think, as if all your beautiful old MF lenses find their place in your camera bag. This can make you feel a little better, despite the high price.
The reason why I did not give a 5-star is half point from the lack of AF and another half-point for the high price.
I highly recommend getting one if you are into the beauty and the quality of the old medium format lenses.