Just purchased the 150-500mm Sigma for my wife's Nikon. I felt compelled to test it against the 3 similar lenses we have. Here are the results, in order of image quality:
At 300mm, f8:
Nikkor 300 f/4
Nikkor 70-300 vr
Nikkor 55-300 vr
The 300 f/4 and the Sigma were very close. The Sigma's image was sharper, though. The 55-300 was noticeably less sharp. The others were close.
Sigma was noticeably sharper than the 300 f/4 with a 1.4xTC. Not enough to make a difference in the real world. But, when testing the OS on the Sigma it quickly became the real winner. The above tests were on a tripod with no image stabilization on. When hand holding the Sigma my wife shot several shutter speeds: 500, 250, 125, 60, 30, and 15. I still cannot believe that we could not tell any difference between the shots taken at 1/500 and 1/30th. Even when blowing the images up in photoshop until individual pixels were showing. The Nikkor f/4 does not have image stabilization, so I am now thinking of selling it and getting another sigma.