Last weekend I was out at Firebird International Raceway for the NASA-AZ event. Despite scalding temps (90 degrees in the spring? Hotter over the pavement of course) the lens worked fine all day long. My D300 was having an intermittant power short problem in the connection to the lens, but removing the mount ring from the body, tightening down a few hidden screws, and reinstalling the ring seems to have corrected this issue. I couldn't make my Nikon lenses do this but they aren't nearly as long or as heavy as the big Sigma.
Nest weekend is the start of my "crazy season", with Formula Drift Long Beach kicking off the year in style. The image stabilization works for panning, somewhat, but as with all OS systems, they're designed to pan with humans, like runners, and not cars. I'll be taking my monopod and using it a LOT, as that's simply the best way to get a good photo, sharp photo.
In sunny, high contrast conditions the lens resolved details down to the carbon fiber level on passing cars. Posed shots show fantastic detail, almost to the level of my 35mm prime lens. In lower light, it was fun to shoot at f4 and f2.8 for the background bokeh. The Tamron lens was a little better for color and smooth bokeh, but there's nothing objectionable about the Sigma's output at all. It will be a fun time in Long Beach Friday and Saturday.