Sigma 170-500 Good? Bad? Your experiences

I found a great deal on a Sigma 170-500 for my Nikon d200 and was wondering how others liked or disliked this lens. I've heard that it is a bit soft at 500mm and the focus is not lightning fast, although this may vary with different bodies. Primary use would be birding while on a tripod. Some BIF shots, but  mostly stationary.