CC Chang , Apr 21, 2011; 01:45 p.m.
Just to remind you that both lenses have been discontinued, and a new version of the Sigma with OS has been announced. So you may want to wait to get the newest thing ... Newer 3rd party lenses may work better with newer camera body in terms of AF consistency.
Having said that, I did manage to find a copy of the Sigma to use on my D90. It does front focus and after being repaired by Sigma, it still does but to much less extent. When I called the Sigma, they said that they could adjust the lens to fit my camera, but they mostly recommend this to people who live close by as it may take a few trips to get it right. This was not an option for me, however. I also learned from talking to them that the equipment they have to calibrate the AF is designed to check at only 50 and 150mm, but not any where in between. So it is possible that while a lens is calibrated "properly", it can still have some issue, most likely at focal length different from the two extremes. Despite the small front focusing issue, which is evident when I tested in at night in my kitchen at about 70-100mm, it seems to be field irrelevant as I don't see this in most of the actual pictures I took when the light is relatively good. The lens is sharp with good bokeh and small and light for the fl that it covers. The "softness" that some people reported may be caused by inaccurate focusing, but not the glass itself. I heard fewer complains on the Tokina. If you plan to use this lens a lot indoors at close range, Sigma's AF accuracy may be a concern.