Links

Reviews of the Sigma 500mm f/4,5 EX HSM:

User reviews on Fred Miranda
In-depth review by Jason P Odell on Nikonians.org
Shutterbug magazine (take with a grain of salt because they hardly ever write any negative reviews)
Popular Photo review (take with a grain of salt because they hardly ever write any negative reviews)
Test by Photodo including images and MTF curve
Nature Photographers Online Magazine review
Digiscoped.com review