Also, here is another very very positive review from E.J. Peiker at Naturescapes:
Pricing probably varies globally.
The Canon 500 is around AUD 11,000 to 12,000
The Nikon, I have no clue. I would say about a grand more.
The Sigma, I can buy for just over AUD 6,000 so it is a huge saving and I may need to drop my Canon Snob title if I go with a Sigma. As long as it is as good as a Canon, because for me image quality is more important than a cost saving. Even if it's half the price of the Canon equivalent. I guess I could always get a used Canon 500 II in which case, they are around the 8-9K mark.
It would be an honour to endorse a third-party product over the ridiculously priced Canon lens, provided the Sigma is an ass-kicker.