My experience with the f:2 Heliar is the same as Paul's (photophorus). A little soft wide open, sharp when stopped down at least one stop. I also own the f:3.5 lens in Nikon-S mount which I use on my M's with an adapter. It is razor sharp at all apertures, and has been stated elsewhere, has no click-stops in its Nikon configuration. You can look at the performance of a lens like the 50/f:2 Heliar in one of two ways: either you have an f:2 lens that has an f:2 soft-focus option (sort of like a Canon 50/1.5 or an old Zeiss 50/1.5 or Nikon 50/1.4 wide open) and is reasonably sharp after that or you have a design that is really only an f:2.8 design, which happens to open wider. Since most of my 50mm lenses are dead-sharp by f:4, I view the F:3.5 Heliar as less of a triumph, although very impressive if its look is what you are after. If I had to choose one, I'd choose the f:2 lens, as I always prefer more options.
Ben Marks
