Excellent point Doug re: central focus point. BTW, I also have a 300mm f4 IS L (one of my many lenses) and it is a bit soft too. My fault for not testing it properly when I originally bought it many years ago. The original non IS is a better lens, sharper too btw. Artie Morris (famous bird photography,
http://www.birdsasart.com/) does talk highly of both the 300mm f4 non IS and 400mm f5.6.
Thanks Humayun for the kind words :-)
Perhaps Chris got a dud lens, it does happen. I'd like to see the lens in action before judging it. Many retailers do tend to fob you off when you report a problem, rather than provide quality customer service and assist the customer.
One thing I meant to mention in my original post - did you buy from an authorised Australian dealer? From memory, Sigma lenses are distributed by CR Kennedy in Australia. There are many online places that sell Sigma lenses, but they are NOT authorised Australian resellers. They do NOT carry an Australian backed warranty. You are at the MERCY of the retailer when it comes to warranty issues. True, that retailer must provide full warranty as per the Trade Practices Act 1974 legislation, but that won't stop many of them from trying to avoid their obligations.
Good luck, and as I said earlier, the offer stands for trying your new lens out on my Mark IIn. Hell, I'll bring my 70-200 f2.8 along and you can try that on your 450D ;-)
Dave
PS Humayun you have a very nice lens selection I just realised ;-)