I started with a Sony NEX-6, and have since moved on to a Nikon D610. But I am still holding on to the former as a secondary or backup camera. However, a secondary camera that doesn't share lenses with the primary camera is of limited use. Luckily, the Sony NEX-6 is a mirrorless camera that can accept Nikon lenses via suitable adapters. The reverse i.e. mounting Sony E-mount lenses on Nikon DSLR is not possible though.
I decided to purchase the Metabones Nikon G Lens to Sony E-Mount Speed Booster ULTRA to attach my Nikon F-mount lenses to the NEX-6. I like the idea of using my full frame (FF) lenses on a cropped sensor camera without worrying about the crop factor. The speed boost of 1 f-stop and improvement in image quality simply make this adapter a no-brainer.
Of course, reality is never that rosy. I can't autofocus with this adapter, which is not a huge loss since NEX-6 has focus peaking and manual focus (MF) assist (need to enable Release w/o Lens to work). I also will be primarily using this adapter for landscape and macro, which suits MF anyway.
Next, there is no focal length, aperture, and lens identification info transmitted; I only know the ISO and shutter speed. Hence if the missing info is important, I will need to record it down manually somewhere.
Last but not least, Tamron's vibration compensation (VC) doesn't seem to work, at least for Tamron SP 24-70mm F2.8 Di VC USD Nikon. Again, not a big loss and can conserve battery life to boot!
Now let's talk about some positive things. The increase in light gathered by 1 f-stop is real; too bad the depth of field (DoF) doesn't drop correspondingly as well. In other words, an f/2.8 lens will become f/2 for exposure purpose but the DoF is still f/2.8. But if my understanding is correct, this is already an improvement because normally there is a need to multiply the DoF by the crop factor to obtain the effective DoF i.e. if I am not using a Speed Booster type adapter, the f/2.8 lens will actually provide f/4.2 DoF even though the light is still gathered at f/2.8.
I looked into hyperfocal distance calculation with this adapter as well. I believe one should use the reduced focal length (by 0.71x) and a cropped sensor's circle of confusion (0.02mm) to compute. But since that will consistently yield results that are shorter than the FF focal length and FF sensor's circle of confusion (0.03mm), I can just use the FF calculations if that is more convenient e.g. the DoF scale on my AF Nikkor 50mm f/1.8D.
Another thing I like about the adapter is the removable Arca-Swiss foot. Although it looks a bit too slim for comfort, it does secure to my tripod head.