I just picked up one of these today, for $650 I couldn't pass it up. Just from the very non scientific tests I've done between it and my Ultron 35mm f2, the Summarit looks really nice. I never noticed how much the Ultron vignettes wide open until I tested it back to back. You may lose a half stop in aperture but with the reduction in vignette they feel pretty similar speed wise.
I had the 7artisans 35mm and it looked amazing with b&w film but it didn't do well with color, the distortion was pretty bad, the flaring was horrible and the build quality wasn't very good. The Ultron was better in every way, I just miss the focus tab (the stick is okay), the build quality is loads better (better than the Summarit IMO) but the slightly swirly OOF sometimes isn't that flattering. The main reason I wanted to try something else was the Voigtlander just has a little too much contrast. Hopefully the Summarit's contrast is smoother with pushed b&w film and takes some of the edge off of my M10's digital'ness. It's looking good so far though.