I just purchased this teleconverter to be used with my 100-400. After reading many reviews I decided it was worth a shot. I'm happy to say that the AF works in many situations. I did some testing today in my back yard from my tripod. The Goldfinch has been cropped down about 75% but even in cloudy conditions, it locked in fast and accurately. (not tack sharp but it is a huge crop). However, even with the sun shining on the dove, it would not stay locked in and I had to switch to manual focus. As you can tell, the finch has a lot of contrast where as the dove, which was quite a bit closer and cropped only about 50%, has very little contrast. All in all, I'm happy with this purchase and just have to remember it's limitations.
As a side note, I don't normally take feeder pictures. 
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