Does anyone have some real world feedback? How well does it perform, image quality, autofocus issues, etc. I would like to know if using crop mode with a Nikon 70-300 VR will give me a "close enough" image vs buying a Sigma 50-500 or 150-500. I currently have a D7000 and the 70-300, the Sigma or the D7100 both fall into the same price range so it's a toss up on which to get. If the crop mode produces decent pics then I could get the d7100 body and shoot with my current set of lenses. If not the I'll pop for the Bigma and hold off on the 7100. Thanks in advance.