Hi, Jason. By now, you might've already sprung for the purchase, in which case you're probably a happy camper...unless you're a pro, who needs the new 400 or 600 w/ built-in TCs. ; )
In case you're still considering the combo, I do have the 100-400mm and the 1.4x TC, and the results are very good. There's no degradation in AF speed, and you really have to pixel peep to see a difference in image quality. If you're shooting for personal pleasure, to share pics online, or even to occasionally print something to hang on your wall, you'll probably be very pleased with the results. The 1.4x was my first TC, and I'm glad to have it in my bag. I'll likely trade it in one day, toward the purchase of the upcoming 200-600mm, but that's the only scenario it'll happen.
There's a wealth of helpful YouTube videos on the combo. Here are a few:
Hope this helps you.
