The Sigma 150 - 500 is not my first choice for birds in flight. If I know I'm going for in flight photos I usually use the Nikon 70 - 200 VRII.

The Sigma autofocus speed is slower than the 70-200. Accuracy of initial focus is good, but if it misses, it's hard to gain focus after that. Once it locks on, it will track fairly well. And, of course the extra magnification makes aim and panning more difficult.

Some of this depends on which body you're using and how much practice you've had.

All that said, I use it for birds in flight when it's what's on the camera - and it works. A couple of examples (sorry no D7000 yet):

http://www.flickr.com/photos/edrosack/4376168179/in/set-72157623223995224/

http://www.flickr.com/photos/edrosack/4467275213/in/set-72157623223995224/

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