I've seen some fabulous images taken with this lens, I don't own one but reports from owners and the images has covinced me. If I can justify the cost (compared to others the sigma is a bargain) I will have no hesitation owning one. IMO, it is a fabulous lens.
I've heard it doesn't work well with TC's, IQ deterioration is too much.
A birding lens could definitely use the HSM, screwdrive sucks for this, but what do you do?
What's the other fast mystery lens you have

, is it the sigma 300/2.8....what do think of that one?, I've heard mixed reviews for that...post some images.