Now that Dpreview and Photozone have both had a chance to take a look, here’s a mix of results from “Sigma Fifty” and its Canon/Nikon competition. These are the FX results from Dpreview. I’ll briefly mention DX (incl. Photozone's) results below.
Wide open the Sigma Fifty pretty clearly dominates. This is what it was designed to do and why it’s such a big bucket of a lens. The poor performance in the corners is almost certainly due to a 2-stop (!!) light fall-off (colloquial vignette). Don’t be too bullish about the Canon or Nikon either, they have 2.7 stops of fall-offs.
The Nikon lags throughout the test but here at f/1.4 the gap is just staggering, especially when you consider that the Sigma/Canon are paired to a 21MP 1Ds-III and the Nikon is on a 12MP D3.
Both the Canon and Nikon improve tremendously around f/1.8 but the Sigma continues to rule the pack through f/2~f/2.8. The Sigma’s center resolution here is just enough to quench the DX format over most of that frame. Light fall-off is now ~1 stop for the Sigma and ~1.5 stops for the Canon/Nikon.
Stopped down all three are solid performers, but the Sigma is at its limits on the 1Ds Mk-III. Somewhere around f/4 the Canon comes out way ahead. It looks like the Canon 50mm f/1.4 has resolution to spare up to ~30MP beyond this aperture. The Nikon has just a bit more resolution still in it, but recall that it’s sitting on a 12MP sensor only here (but colors are normalized so it’s still apples-to-apples there). Light fall-off is nothing to worry about from f/4 onwards for any of them.
What’s the DX performance like?
It’s a mixed bag. Used stopped-down the Sigma Fifty is just good enough to quench DX sensors, and at any aperture has even behavior across the whole frame without any light fall-off. Wide-open it’s not quite as sharp as the Sigma-30 however.
Comparing the results of the Sigma .vs. the Zeiss in the Photozone tests (note: crossing shooting platforms hurts comparison) I believe they are roughly equivalent for most purposes. I would select the Sigma for center-axis sharpness at f/1.4 but probably pick the Zeiss for anything else.
A word about the Bokeh
Back a few months when this lens was announced I was a bit worried about the bokeh, because it's typically not Sigma's strong suit. You’ll want to check out this small collection of images then: http://www.motleypixel.com/reviews/index.htm Certainly that’s not bad. Based on these and other images (on Flickr) I conclude that the bokeh is marginal at f/1.4 but improves suddenly at f/2. From f/2 onwards the bokeh is soft and “relaxed”, I can’t find anything bad to say about it.