If you guys like the mamiya 300mm, you'd probably love the Schneider/Rollei 300mm. I had to go back through my image catalog and compare some images taken with the Schneider 300mm f/4 for Rollei against these images taken with the Mamiya 300. I knew I didn't like look of those mamiya 300mm images but it was hard to put my finger on why exactly. After comparing images I have a better idea. The bokeh of the Schneider is very smooth and not distracting. The mamiya has hard edges and a strong look to the bokeh which competes for attention. But why would you want the background to be competing for attention if you are using a fast lens to isolate the subject in focus? The schneider renders both foreground and background OOF areas or bokeh significantly better - smoother with no hard edges or double edges (all of course IMHO) than the mamiya. Plus the mamiya suffers from specular flare - take a look at the large floating circle images of the aperture shape in many of Brian's test shots. I guess it's a personal opinion but I don't like those large 'bubbles' at all.
I went back to Brian's images again and examined every one closely. There is NO evidence of "specular flare". The "large floating circle images of the aperture shape" are not lens flare artefacts; they are real scene features (patches of light filtering through the leaves) rendered oof.
But I won't argue whether the 300/2.8 bokeh could be nicer - it could be.
It's great to hear that the Rollei lens is so good, for the Rollei/Hy6 platform. It's just a pity that those lenses can't be usefully adapted to other systems (the flange distance is fine, but there's no way to control the aperture...so not even the Chinese have come up with an adapter).
But hey, if you want a 300/4 lens with less distracting bokeh, and you shoot with a Mamiya (or Pentax or Contax), all is not lost! It's hard to beat the bokeh - and you certainly can't beat the price - of the CZJ MC Sonnar 300/4. Image quality in the plane of focus is also very uniform, right into the corners.
Below is an astonishingly boring photo that I took on a dismally grey day with the 300mm at f4 (well, I sought out another "tree in front of trees" photo to match Brian's, and this was all that I had taken with the Sonnar). Anyway, I show it here because I think that the bokeh ain't bad.
That said, the 300/2.8 is on my "some day" list to replace my 300mm Sonnar, because it is f2.8 and a full-on APO. Rear bokeh just doesn't come into the realm of astrophotography
And my 200/2.8 makes a super, portable telescope, so the 300/2.8 would be even better: in visual astronomy, linear aperture matters bigtime, and 107mm beats 71mm handily - good big 'un beats a good little 'un, as they say.
Ray