I'm a Nikon user, and got my Sigma 17-70/2.8-4.5 (first version) when bought my D300.
I was really happy and amazed of how sharp could be that lens, and even I got the Nikon 16-85 VR only to sell it some time later because I liked way more the microcontrast and clarity of the Sigma.
Then the 17-70 OS version showed up, and being so happy with the first version I bought this OS one. But I was shocked because results weren't so good. Tried three copies, and below 35-50 mm field curvature was really a problem even at f/8: focus on the center and get blurried sides, focus on sides and get blurried center. It was so heavy that I almost couldn't believe it, but I was seeing it... A couple of years ago I could test a new sample from a friend that didn't show that field curvature (???), but anyway results weren't that good, my old 2.8-4.5 was still better on corners closing at f/5.6 than the OS at f/8.
And now here comes this C version, and again I'm quite excited about it. Well, not that much now, because when comparing measurements of the three versions at 17 and 24 mm I can see that the first one still delivers sharp corners, but not so the OS nad C versions.
Can wait for real samples, but I have liked to see something different at the wide end in the corners.