I have had the Sigma 70-200 F/2.8 and sold it for the Nikkor 70-200 VR. I use my fast lens's for sports photos' mainly motor sport, so sharpness is everything.
I justified selling my Sigma as I found I wasn't happy with the results. However, like a fool, I only gave one real opportunity to prove itself, in very low poor light, shooting rally cars in the rain. It didn't do well, and soon found itself on Ebay.
With hindsight, I think it was a multitude of problems, including shallow DOF, poor quality filters that caused ghosting in high contrast images, the D70 not being a D2x and possibly also a very slight back focus issue with the lens when wide open.
That being said, I can still produce less than quality images with the Nikkor VR. I think Sigma gets a bad wrap due to previous QC issues. I think all lens manufacturers produce bad lenses. Ask around here about the Nikkor 24mm-120mm VR. I am happy with mine, but some people want to smash theirs.
I am in the process of possibly going back to Sigma, a 120mm - 300mm F/2.8 , and selling the Nikkor to fund my habit.
http://www.dslrusers.net/viewtopic.php?p=194917#194917
Don't discount Sigma lenses. They are good quality and if you are stuck to a budget, then at the very least they let you get the focal lengths you want. If you buy from a reputable dealer, then any problems you may have (like any brand) shouldbe able to be fixed.
2x D700, 2x D2h, lenses, speedlights, studio, pelican cases, tripods, monopods, patridges, pear trees etc etc
http://www.awbphotos.com.au
