Everybody talks up a streak about how the Sigma 50mm/1.4 is the best thing since sliced bread. I tested it and was underwhelmed on both a D300 and a D700. I've owned a couple copies of the 50/1.4D and picked up the 50/1.4G as soon as it came out.
My experience has been that wide open, the 50/1.4G was the sharpest of the lot, especially in the corners. Dpreview's numerical/color coded test charts agree with what I've seen, but their review seems make some comparison's that talk up the IQ of the Sigma compared to Nikon and Canon. Their quantitative analysis and sample images contradict a lot of the positives bestowed on the Sigma in their review.
Anyway, I'm happy to see photozone come out with a full frame review which reaffirms what I've been saying all along: the Sigma doesn't hold a candle to the 50G in terms of sharpness: http://www.photozone.de/canon-eos/522-sigma50f14eosff?start=1
I'll admit, I like some Sigma lenses. I'm more than ready to root for the underdog. But I really don't understand why people go on and on about this lens. It's a dud! Get over it!
The Nikon is cheaper and better. Lemme qualify that: the Nikon is has higher resolution, better corner sharpness, better weather sealing, it smaller and lighter, and is easier to manually focus.
I love my 20/1.8...
...but really, what's all the Sigma fan-boy-ism about?