I started with the 100-400 but it is now retired. On mine the images were soft. I was happy with it on film but not as happy on digital (even with focus adjust). I found I almost always used it at 400. Other things I did not like led me to look for other options.
I now use the 400 f/5.6 for my walk around birding lens. Light weight, quick, and sharp. Don't really miss the zoom, sometimes though, but that's when I'm too lazy or uninspired to change the lens. I do miss the IS though, it is nice to have in the woods and at dusk or dawn. Using the tripod more now with this lens and experimenting with the 2x for 800 f/11. Using live view for manual focus (you can auto-focus in live view too thought it's slow and not real accurate).
I also have the 70-200 f/2.8 ii and the 1.4iii and 2.0iii and both produce wonderful images. I like having the IS. Not crazy about the weight and balance with the 2x in place. Also have found that it is very easy to move the zoom ring until you actually want to when wearing heavy gloves. I just started using this combo and on a recent outing, in sub-zero wind chills, I found the zoom was always moving and most of my shots ended up below 300mm. I would notice and reset to 400mm but with the gloves it was hard to move and keep the zoom at 400.
No need to worry about image quality with the Canon Mark iii TCs with the 70-200 f/2.8.