Great deal! Wish it worked with my 800 without needing a hardware fix.
I have a 800mm and it seems to work okay with my 5D III. I've not sent it in to Canon as it seems to function normally. I've heard of some issue with the IS and 5D III, but not sure I am experiencing any of the reported problem.
Cams: Canon 1Dx, 6D, EOS M
Zooms: 17-40mm f/4L, 24-70mm f/2.8L II, 70-200mm f/2.8L II; Primes: Sigma 35mm f/1.4, 40mm f/2.8 STM, 50mm f/1.2L, 85mm f/1.2L II, 100mm f/2.8L, 135mm f/2L, 300mm f/2.8L IS II, 600mm f/4L II
Support: Gitzo GT4542LS/GT2830/G2258, RRS BH-55, Wimberley WH-200