I used a borrowed 600/6.5 a couple decades ago. While it was actually quite
sharp and optically well-behaved, the lens itself was a disaster to use for
1. Stability. The slightest bit of wind or vibration would set it off. The
way it balanced with the camera body on the tripod meant that it would rock
or oscillate. Absolutely impossible to keep it still.
2. Vibration from the aperture-actuator mechanism. It set off a resonance
in the camera/lens combination that wasn't possible to avoid.
3. Focus mechanism. The rack-and-pinion focus was absolutely the most
absurd focus mechanism I've ever tried using. I've used RaP focus
mechanisms on other camera systems, but those seemed better balanced to the
task. The thing I hated most is that while focusing, the camera moved.
When coupled with shooting flash, the results are staggeringly sharp. But I
got maybe two or three sharp non-flash pictures over the time I had the
Themed Olympus Photo Exhibition: http://www.tope.nl/