I ordered this lens from Amazon, and was very disappointed to receive a grey import copy, which meant it wasn't covered by the 3 year UK warranty. But I was about to go on holiday so decided to keep it.
It is very compact, and though fairly heavy at 565g feels lighter, sitting unobtrusively on my Nikon D7000. It seems very well made, and the zoom ring feels just right. It auto-focuses quickly and silently, and is spot on, the most accurate lens I've used. However, the focus ring turns when using AF, which is very poor for a lens in this class.
The lens takes very beautiful images, full of colour, detail and impact, and it's very sharp. At F2.8 the image is sharp from top to bottom of the frame at all focal lengths, but soft at the sides. I don't see this as a problem, as at F2.8 you'll be trying to isolate your subject from the background. Closing down to F4 sharpens everything up, but corners remain soft at all apertures. Though there's visible distortion at 17mm, the images are still lovely.
It focuses down to a little closer than a foot, and it retains all its sharpness. At 50mm and F8 it takes the most beautiful, colourful, detailed close-up images of any lens I've used (though note I've never used a macro).
Wide open, there's visible vignetting at 17mm, but also surprisingly at 50mm. Although still very usable, I feel that this is the biggest weakness of this lens. Stepping down resolves the problem.
You need to use the hood to avoid flare, but it's well controlled when fitted.
Optical stabilisation seems to work well, though I don't think it's necessary for a 50mm F2.8 lens. And it really does drain the battery, much more so than other OS lenses I've used.
However, while on holiday after owning the lens for less than 5 months, I noticed that auto-focus sometimes failed to engage. Flipping the AF button to manual and back would resolve it. When I got back home, I found that all the pictures, particularly at 17mm, were soft. The lens had suddenly started front focusing. Setting in-camera micro-adjust to +17 (max of 20) improved it, but the lens just wasn't as good any more. I Emailed Sigma UK to get it fixed, who advised me to return it to Amazon as it was a grey import (Bad Amazon). I contacted Amazon, and they said they'd give me full refund if I returned it (Good Amazon).
3 stars might seem a bit generous for a lens that lasted for less than 5 months, but it did take lovely pictures. And I am thinking about buying another as it still seems like the best all-round 17-50, though not a grey import this time.