My impression of the DPR lens review of the Sigma was that the Sigma wasn't nearly as sharp as the Tamron or Nikon lenses. I admit I haven't chased down the MTFs though. In fact, if Tamron has fixed AF speed, or if the Tamron AF for Nikon is better than it is for Canon, it seems like the better lens overall if you're looking for a reason to not buy the Nikon.
On sharpness you are right about their tests, but the Tamron's AF was much slower. It was a wash and it depends on what the person's needs are. If you are shooting sports then AF is more important at least to me it is. The pic quality from the shots I took with the Sigma were great. I wanted it to perform noticably worse than my Nikon because I paid a lot more for my Nikon, but it wasn't a huge gap overall in my tests. Not at all.
All I know from my own real life tests is the Sigma is a heck of a deal and is a very nice lens overall. Its AF is very fast and pic quality is good.
As I said I own the Nikon 70-200 VR and while it is nicer than the Sigma it isn't $1000-1200 nicer. Nobody who is objective would say it is worth that much more. If they do they own the Nikon 70-200 and just want to claim that to feel it is to justify paying that much, they have never used the Sigma (honestly never used it), or they are a Nikon fanboy. I only own Nikon lenses (that is about to change on my next purchase), but I'm objective and fair(some tests I have done on various friends 3rd party lenses opened my eyes). There are a number of 3rd party lenses just as good or better than the Nikon equivalent. In this case the Nikon is better, it just isn't that much better for the price.