Everyone seems to have their own opinions which makes difficult to
obtan an objective comparison. Professional reviews are often the
best but not necessarily flawless.
Popular Photography reviewed the Sigma in September 2004. You can
see the review here:
http://www.popphoto.com/article.asp?section_id=2&article_id=1111
Popular Photography also did quite an interesting VR comparison.
Unlike most tests on this forum where a few shots are taken and a
call has to be made, they used several (experienced) photographers
and a total of 3,500 shots before drawing conclusions. The results
of the the test are available here:
http://www.popphoto.com/article.asp?section_id=2&article_id=1039&page_number=1
They concluded that the additional effective stops obtained are 1.0
(80mm), 1.3 (250mm) and 2.0 (400mm) for the Nikon. The Sigma OS
delivered 1.0 (80mm), 1.6 (250mm) and 2.0 (400mm). Clearly no
significant difference but a slight advantage going to the Sigma.
I can concur with Wally Oz's comments on fast focussing of the
Sigma. The Nikon uses the internal camera motor and is therefore
dependent on the particular body used.