Currently, I occasionally use the 1.4x tc with the 100-400, and not being able to autofocus does not really bother me, since I don't have a 1 series body. Most of the time I use the 1.4x tc is only with my 70-200.
FWIW, I tried a comparison of the extenders on the 100-400mm vs. cropping the image (using the 7D), and with the 1.4x, I preferred the cropped image, whereas with the 2x I preferred the extender.
Oh, that's interesting. I'd assume the 5D2 would be better cropped as well, as opposed to using the TC.
My favorite use, albeit not very useful--just fun, of the 100-400 with 1.4x (I don't have the 2x) was getting super long distance shots with that combination + an xl-h1s for surveying things. This is a 560*7.2 crop factor= 4035 full frame equivalent. Yes, it was blurry; yes, it was crazy hard to hold steady, even with a tripod; however, you could see objects from quite a long distance away.
Add the xl-1.6x tc and you get a 6456mm full frame equivalent.