Upload & Sell: On
The Mamiya A 300/2.8 APO exceeds my expectations in every respect. It has extremely high resolution, moderate to high contrast, moderate colour saturation, no vignetting, and no CA in even very challenging conditions. As Conner999 said, it's well balanced. Also, it's extremely well made, but not weather sealed. I've used it with a simple rain cover made from a sliced-open kayaking dry bag in very bad weather, so it's not really a problem.
Using the USAF 1951 test target from 13m distance with LiveView on a 40D, it reaches 71 line-pairs per mm (lpmm) in the centre at f/2.8, and 80 lpmm at f/8, with 71 lpmm at the top of the frame, at f/2.8 and f/8. My 300/4L IS achieves 66 lpmm across the frame at f/4 and f/8 on a 20D using AF from about 7m. I consider my 300/4L IS to be plenty sharp, and so the 300 APO is a wonder. My 200/2.8 APO achieves 66 lpmm in the centre at f/2.8 and 74 lpmm at f/4, in the same test conditions as the 300 APO. I made the mistake of handling conner999's 200 APO - I can hardly wait until he blames me for getting a 300 APO .
The 300 APO works equally well with the Canon EF 1.4x Extender and Kenko Teleplus Pro 300 DG 1.4x TC (both are slightly better than the Tamron AF SP 1.4x). The Canon EF 2x II Extender seems to be slightly better than the Mamiya 2xN TC, but I use the 2xN most often, as I don't want to modify the EF 2x to get it working with the 300 APO on a 1DII (you have to grind down one of the mount flanges so it doesn't hit the AF microswitch). The 2xN is also wider and seems to be stronger that the EF 2xII.