How much better IQ will a person get out of the Nikkor 24-70 2.8 compared to something like the Sigma 17-70 2.8? Is the IQ really that much better? I realize the build quality is far superior but I am curious about why people think so much of this lens. I am seriously considering purchasing it and would appreciate some hands on experience from owners of the lens. Some image comparisons would be appreciated from similar focal length lenses if anyone has a few handy.