I had the opportunity to test "Bigma" on the Nikon D200 this morning at the park :)
Unfortunately the weather was not good (very cloudy). At 500mm you do shake and if the light isn't good enough you get many blurry photos. To avoid the shake it is better to take photos at 1/500 second and faster which means increasing the iso (and noise).
The lens is a little heavier than the Nikon 70-200f2.8 but I did not see any difference. It is a little long and not well balanced compared to the same Nikon.
I thought the autofocus speed was decent and the reach wonderful. The problem is the lack of image stabiliser but the new Sigma lenses coming soon have it so they can be interesting.
It is the kind of lens that makes me want to get a 500f4 prime lens :D

Two photos taken at 500mm f8:
A robin
http://www.photoboard.eu/photography/showphoto.php?photo=2655
A parakeet
http://www.photoboard.eu/photography/showphoto.php?photo=2656

Very nice and detailed images!It wouldn't easy for me to handhold a so heavy lens and take clear photos!

Actually it is not that detailled in 100% crop but then if we want better we need to get some 500f4 prime lenses and that would be 4 times its price :eek:

Here is another one I took with that lens:
http://www.photoboard.eu/photography/showphoto.php?photo=2661
Not sure about the bird species.

We have many birds like this one in Greece,i think the name is goldfinch.

You are right, it is a finch :)