Metabones was the first to announce this revolutionary focal reducer (0.71X) for mirrorless cameras as posted here some time ago. They were first also to announce a Nikon F version of it, but since the release was delayed (maybe we will see some at the end of this summer) some competitors entered the market with comparable (0.72X) products. Mitakon first, then Fotodiox with the Vizelex version, both offered at 150US$/€ (so less than half the price of the Metabone's).
Here are the first reports:
The Mitakon is said to have a lower optical quality. Amateurish.
Fotodiox failed the opportunity totally, with an unusable product. Many customers complained, and Fotodiox announced an updated version of the reducer with an improved optical element (priced at 200US$/€ with free replacement for early customers). They also announced a 0.5X version in M4/3 mount, but we don't believe to this assertion (too good to be true).
UPDATE - July 5: New Speed Boosters are now available from Metabones in several mounts:
Nikon (G) to M4/3 and to E-mount
Leica R to M/43 and E-mount
Contax/Yashica to Fuji X, M4/3 and E-mount
Prices ranging from 400-430US$/€, not cheap but 200 less than the Canon EF version.