Are the Sigma telephoto lenses ok? (especially autofocus)?

I have a friend who owns that lens, his images with it are stunning... he uses it on an XTi and an XT... the photos are sharp and clear, he uses it mostly for wildlife photography, and has on occasions used it for sports. a href="http://chiller.smugmug.com/" rel="nofollow"http://chiller.smugmug.com//a