Andrew Reid has written a decent article on his experiences with the Metabones Speed Booster and the phenomenal Sigma 18-35 mm f/1.8 lens on the Blackmagic Pocket Cinema Camera:
In short I absolutely love this combination and it will be getting a lot of use. There’s not a single moment when I’ve thought something would look better on a GH2.
The soft corners of the E-mount Speed Booster are no longer with us. They faded with the Micro Four Thirds version and they’re essentially here absent because of the 3x crop sensor.
That sensor becomes a 2.1x crop making the Sigma 18-35mm F1.8 a very tasty 38-75mm F1.2 constant zoom. Brightness is absolutely constant throughout the zoom range, although bokeh is smoother at the 35mm end. That’s because the iris blades do not fully open between 18-24mm, to maintain the constant F1.8 aperture. I suspect Sigma have actually created a F1.2-F1.8 zoom here. An extraordinary technical achievement especially for $850.
He provides samples (and video) so you can check out the results for yourself.
He also compares the Sigma 18-35 mm with the Voigtlander 25 mm f/0.95:
Reid the full article here.