I will be extremely interested in a Sigma 500mm f/4.5 (or even better if f/4) OS lens since I briefly tested the 120-300mm f/2.8 OS and it was quite awesome except for the humongous size. I ended up with the 70-200mm f/2.8 IS II for portability but the price was only a little cheaper than the Sigma and reading the reviews the IQ of the new Sigma is supposed to be very nice.