I recently got a 12mm lens for the Leica M8. Leica don't do a 12mm, so I went for the Voigtlander 12mm. It's not made quite as nicely as the Leica lenses (and I've only got the cheaper Summarits) but it still feels like a real old fashioned bit of camera kit.
On the M8 the lens performs like an 18mm due to the 1.5 crop factor.
It gives very nice wide angle shots, even with no coding on the lens.
I finally got it coded as a wide angle lens by putting a black blob in the correct position, in anticipation of the arrival of the 77mm(!) IR/UV filter. I bought the wrong adapter (this is a screw mount lens) and the coding doesn't work when the wrong frame lines are brought up. After I'd bought the right adapter it coded first time.
The filter is on back order, but to be honest I'm not sure it'll make a huge amount of difference, but I thought it'd be worth having one just in case.