Okay, AF accuary is an issue with the Sigma in low light conditions, it sometime misses AF completely then. But as I needed an fast AF lens for sport fotography, the Sigma was my first choice.
Even tough I only use the AF to focus where I would expect the action and then switching to MF to take the actual shot, so that I don't need to press the shutter release all the time to keep the AF locked, while waiting for an other rider (in the case of shooting Mountainbiking). But if I have to adjust the focus in a short matter of time, a fast AF comes in handy, and there's where I have some complaints about the nikon kit lenses. Yes they are razor sharp and yes, the accuracy of the AF is spot on in every light condition, but they are sooooo slow.
I really hope to find someone with an 70-200 f2.8 Nikkor lens when shooting somewhere at the Nürburgring for example, to have some first hand experience with the autofocus. Because sometime in the distant future I'm sure I'll want something bigger and better