For most users, this adapter will work perfectly (not talking about the precision here, just functionality.) In fact, it works with all the New FD, FD, and most, but not all FL lenses. An example is the Canon FL 35mm f2.5. The rear of this lens has a raised portion on one side, where the aperture lever is located. See picture below:
Unusual design - half of the rear is raised. Click for larger.
The problem? The aperture arresting pin on the adapter is set about 1.5mm too low; the pin is resting on the raised portion of the lens and prevents the adapter from mating to the lens. You can see the aperture lever on the lens is at least 2mm taller than the pin on the adapter, so there is room for it to go higher. See picture below:
Pin resting on the mount - Click for larger.
I have another FD-M4/3 adapter and it doesn't have this problem. Clearly, one adapter maker did more extensive research than the other. It's a shame. The FL 35mm f2.5 is a nice lens, and I can't use it on the NEX-6, at least not until I get an adapter that can mount it properly.
Good looking lens. Click for larger.
Don River and DVP at night - NEX-6 & Canon FD 20mm f2.8. Click for larger.