First of all: please note that I don’t currently own a full-frame camera (hello Sony? Still waiting for that 50 Mpxl, Nex7 form-factor camera…), so this lens has been tested only on (and specifically bought for) an Aps-c Sony Nex 7.
That said, from the full-resolution images I saw online shot on the A7r it looks still a pretty good glass, just with not-so-exceptional borders.
Second: there are actually two versions of this lens, and old one (the one reviewed here) and a newer model that you can spot instantly because it is bigger and with a front ring to mount filters.
Optically they are exactly the same; the only differences, above the aforementioned possibility to mount filters, are that the new lens is rangefinder coupled (important if you ever want to use it on a Leica) and has natively a Leica-M flange, instead of the Leica screw m39 of the older model (not a big deal, adapters are good and cheap).
The different flange may still be a matter of preferences, though: the new model does not require an adapter for use on modern (post 1950) and digital Leica M cameras. The older model, on the other hand, is more of a jolly that you can use on a modern or digital Leica M with a cheap adapter or directly on an old Leica / Canon etc. screw-mount camera. All depends by the kind of gear you already own or plan to buy one day.
The review
Mechanically this lens is an absolute jewel, almost Leica-quality like, and in my book the archetype based on which all other lenses for mirrorless or rangefinder systems should be designed.
It is unbelievably tiny. You cannot understand simply from looking at the pictures how small it is. I can toss it in a pocket and forget about it, much like my car key.
But at the same time the controls (focus and aperture) are well spaced and easy to grip; wearing heavy sky gloves as well, just with a bit of more trouble. More, the aperture ring clicks positively in each position, almost silky smooth.
However, all this mechanical prowess would mean nothing if the lens should not deliver optically.
The good news is this little one really packs a punch!
Keeping in mind that I’m using it on a Aps-c camera, albeit a really taxing one, it is very sharp up to the extreme borders. Normally with a lens this short (even if on an Aps-c it is really just the equivalent of a 22-24mm) on film you wouldn’t even bother to focus, not even at full aperture. That’s the reason why Voigtlander decided to forfeit a rangefinder coupling with this lens back in the days, to cut costs.
But on digital, and especially on a taxing 24 Megapixels sensor, even at f/8 you can still easily tell when your subject is not perfectly in focus.
Oh, by the way: this minuscule lens is so sharp that at the center it tops the chart at f/5.6, and at f/8 you can already witness quite clearly the effects of diffraction!
If you’re not so sure this is a lens for you, keep in mind that lured by the new model many people is avoiding buying the first one, so you can find one for ridiculously low prices.
If, on the other hand, you plan to use it on a film camera, especially for landscape and b/w films, grab the new one because the filter ring will definitely come in handy.
The only problem with this lens is the extensive magenta / purple coloration at the borders if used on some cameras, like for example the Nex 7. If you shoot in b/w it doesn’t matter, obviously. And if you shoot in color it is easily correctable anyway using the Lightroom Flat Field free plugin (you can get it at the AdobeLabs) or the also free CornerFix. So no big deal in my book, but I’ve lowered the rating half a point just because of that.
Rating: 
on Aps-c
