I've used it for 35mm over the last year. It's sharper than any of the EL Nikkor, Componon or Rodagon 50mm lenses I have access to. I don't have the most exacting of tests; I have a 35mm Rowi test negative that I use to compare lenses and apertures. You can really notice the differences in the "chevron" and "grit" (for lack of better terms) areas of the print. I use a glass carrier and the Versalab alignment tool to keep things in line as best as I can.
It's not a great lens at f3.5, but at f4.0 and f5.6 it's quite amazing. Besides the test negative, I'll make comparison prints with different lenses, and it came out on top every time when I did blind tests. I trust my eyes with this sort of thing. As you'd expect, there's very little vignetting because of the focal length. On my Durst 138s I have to use a very recessed lens board (Latub) to use a 50mm lens, and I suspect that contributes to the vignetting I get when using a 50mm lens. So it's a nice tool in the shed for sure.
As far as coverage goes, I think you could get away with it at 6x6, but only just. Not full frame. My method for checking is two fold. I use an attachment for my Durst 138s called a Larka:
It fits where the neg carrier would go; it was meant for turning the enlarger into a copy stand. You can see exactly where the vignetting becomes an issue when doing this, and use the markings on the scale as a reference. I also use my Darkroom Automations enlarging meter to give me a quantifiable figure. Falloff is a bit more than half a stop on the edges with 6x6 and the 68mm lens - more than you'd like, for sure. As it stands, I have an older Apo Rodagon 90mm for 6x6 and 6x7 which is wonderful.
I think pentaxuser is correct; 40x40mm seems right where this lens lands. But when did they stop making that format? Must have been ages ago. This lens seems more modern.
I've found a use for it, no doubt, but I always wondered what it was intended for in the first place - and why there's relatively little information about it online.