I purchased the 1.4x MkIII TC for my little 100-400 this weekend and took if for a spin. It's definitely a keeper and it allowed me to get some Grebe shots that have been at the top of my 'want' list for some time now.
My, personal, favorite of the day:
Siblings:IMAGE LINK: https://flic.kr/p/Uafz5J Pied-billed Grebes-3722 by Eric, on Flickr
Lunchtime:IMAGE LINK: https://flic.kr/p/VfgnCH Pied-billed Grebes-3672 by Eric, on Flickr
IMAGE LINK: https://flic.kr/p/Vck1pb Pied-billed Grebes-3561 by Eric, on Flickr
Big pile o' cute:IMAGE LINK: https://flic.kr/p/Vck3u3 Pied-billed Grebes-3547 by Eric, on Flickr
Thanks for looking in.