Tim, your high-end background is going to make you unhappy with the AF speed, I am quite sure. It's not glacial, but it's not a sprinter either. For the price, and the optical performance, those of us who find workarounds wind up finding the lens indispensable. But it's not going to pin down something moving quickly time after time, quickly and easily. I've probably rambled already here about my shortcuts, holler if you can't find them, or want any suggestions.
This is the second time this Osprey has flown into the viewfinder while I was focused on the mate and chick. I hit the trigger so hard I'm surprised the nest was that sharp. If I had set my shutter speed a click or two faster I would have a decent pic I could have worked with. I like the wing movement but I just couldn't get the rest of the bird clear enough in post.