This is a beautifully made lens. Fast and contrasty. On my RD-1 APS-C CCD sensor, it has considerably less vignetting (brightness falloff_ compared to its little brother, the Voigtlander 21mm f/4 Color-Skopar. On my Sony a7s, it has much more consistent color across the scene, where the Color-Skopar shows considerable green shift at the edges of the sensor (Note: these cross-frame effects should not manifest on film; its an interaction between the lens and digital sensors). The Ultron is also somewhat more contrasty and has much reduced internal reflections (things that cause flares in the image when pointed directly toward a bright light source). On the other hand, the Ultron is not quite as sharp as as the f/4 color-skopar in the center, even when compared at identical f-stops. The Ultron also produces noticeably more chromatic aberration when compared to the color-skopar at f/8 (this manifests as red and blue tints at the contrasty transitions in the image). How to weigh these tradeoffs? The chromatic aberration of the Ultron is quite easily ignored in most images, whereas the falloff and cross-frame color shift of Skopar is quite noticeable. However, the falloff is more readily fixed in post-process (admittedly, you do some dynamic range in the correction and the photoshop built-in filters don't quite fix the color shift). The cost of not having to do the falloff correction digitally is thus the considerable mass of the Ultron. But when I compare the extra mass to the few extra computer clicks needed to have the deliciously petite Skopar, I choose Skopar. I would only keep the Ultron if I (1) really needed the extra speed, or (2) could not post-process images. So consider the slower 21mm Color-Skopar, it's a sweet little lens that's super sharp and easy to carry.
Other notes: The Ultron seems to be very slightly faster than my Skopar at identical f-stops. It's also potentially slightly less wide than the skopar, but only just. It's easier to focus and handle because the lens is physically larger. The petal-hood is not removable. The lens cap doesn't stay on well. Front glass is protrudes more and will be easier to fingerprint.
This is a fabulous lens, I use it combination with a Sony A7II and the results are breathtaking. The angle of view is just perfect and it is easy to correct for distortion in the latest version of Lightroom.