This appears to be pretty good, for a mirror — which are known for low contrast, ugly bokeh, and low sharpness. I compared it with the Olympus OM Zuiko 350mm f/2.8, and it seems fairly sharp. This appears to be a cut above the crowded field of 500/8 mirrors.

I paid US$177 plus $25 shipping for this one, which was listed on eBay "for parts." I think it went so low because of poor photography: the last photo is what I thought I was getting, the first is what I got! So it sat there without any interest for a month, then I put an offer on it the day the ad was to expire.

3rd and 4th photos show a comparison of this lens with the Olympus OM Zuiko 350/2.8 with a Olympus 1.4X-A matched teleconverter, to give equal field of view.

(For some reason, this won't let me put the eBay photo in. I tried re-sizing and increasing compression, to get it down to a 70k JPEG, and it still won't appear.)