I have been offered this lens by Sigma in Sydney, Australia for a weekend's testing over the next weekend (15-17 December). If the deal goes ahead, I will be writing my thorough user review.
There are at least two credible reviews out there I have read that have given it an extremely good rating, on par with the exxy Canon and Nikon 500/4 super teles. However, no one tested it with a 2x converter. That is my only gripe. There are two reasons for that gripe:
1) My current Canon EF500mm f/4L IS USM (series 1) takes the EF 2x MkII very well and if used properly I am capable of capturing very good quality images, even hand held - albeit I need to close the aperture down to f/10 or f/11. Now, I can even put an EF 1.4x MkII converter on to stack the two together (2x + 1.4) for a 1,400mm f/11 lens and I am capable of taking very good images that are of publication worthy quality (I stop down to f/16 though) under the right conditions using my tripod (for 1,400mm I do use a tripod, but hand hold mostly in other instances). So the Sigma has to deliver good IQ at 1,000mm.
2) Will the Sigma 500mm f/4 DG OS HSM be as good wide open with the 2x as the Canon EF 500mm f/4L IS II USM with the EF 2x MkIII? The Canon has superb edge-to-edge image quality at f/8, thus I would not need to stop down to f/10 or f/11 whilst using this set-up. If Sigma is a match at least at the 1,000mm f/8 mark with the Canon EF 500mm f/4L IS II USM then I am sold! If not, I will pay the AUD 11,500 for a new 500/4 from Canon.