I have the Sigma 120-400mm and am very pleased with it. I will however mention that I don't use the OS
feature due to the fact that it is somewhat unstable. You can see some of my photos on my flickr site: www.flickr.com/photos/avsfan.
This one in particular was taken at 400mm (w/o the OS), A-Priority (f/5.6), shutter speed of 1/500 and an ISO of 200.
[Edited 2008-08-04 13:26:34]
[Edited 2008-08-04 13:27:09]
"Oh I have slipped the surly bonds of Earth...Put out my hand and touched the face of God"