....but seriously. Have you compared any of the ART lenses to Nikon's counterpart and if so, how much of a difference is there, really?
Whether they're worth what Sigma is charging for them is certainly material for conversation but yes, there are significant differences. Most notably I'd say the differences are Sharpness (meaning overall image sharpness) but more specifically, and just as important to me, is sharpness
in the corners and even more specifically sharpness in the corners when shooting wide open and/or at wide-ER apertures (e.g. f/1.8, f/2.0).
Bokeh is another difference but when *I* say bokeh, I'm NOT referring to a generalized, out of focus lens blur. I was taught that bokeh refers to something more specific, that being how
defocused points of light are rendered (link goes to example of what I mean when I say "bokeh").