I had the Tamron 17-50 VC, and was pretty disappointed. Very soft at F/2.8, with a color cast compared to my Nikon lenses. Tossed that and got a 17-55mm, and couldn't be happier. Razor sharp at F/4, and sharp enough at 2.8. Sharpest of my budget trio (including 80-200mm 2.8 two ring and Tokina 11-16mm ).
I also had a Tamron 17-50 VC and I wasn't entirely happy with it.
Now, the Sigma 17-50 OS HSM however....... this lens basically keeps me in APS-C. I would have dumped all my APS-C gear if it wasn't for this. It's a handy focal range in a nice, light package.
It rivals, if not exceeds the Nikon 17-55 in sharpnes, only it's lighter, smaller, cheaper, and has image stabilization. If you want the lens to survive a zombie invasion or a zippy AF, then the Nikon is your goto lens. But in my opinion, the Sigma is a great option if you want to save money, or if you want a lighter kit. The AF is a bit slower, but by now means a slouch.
I voted "other" for the 16-85VR. It's a phenomenal lens. I pair it with a 70-300VR as a 2-lens travel kit, and love it.
I'm planning on getting a newer DX body (sold my D90 earlier, in anticipation of a D7100 in the future), and when I do, I'm also going to look at the new Sigma that's just announced - the 18-35mm f/1.8 zoom. the 16-85VR and 70-300VR can handle most of the things I shoot, however, sometimes, I do wish for thinner DOF, and even 2.8 glass on DX just won't be the same as on FX.... I'm thinking the new Sigma will be just right and promising.
For my more serious shooting, I'm usually on FX.... but the lighter weight of DX gear and the versatility of the 16-85VR + 70-300VR just can't be beat... and if I can get the 18-35/1.8 with it, I'd be really happy. (assuming I get a budget for a new DX body)
F2.8 provides 4x as much light to the autofocus sensors as lens like the 16-85mm f5.6 VR lens. That makes a huge difference in whether the camera is able to autofocus in low light situations or autofocus at all. F5.6 is fine for bright sunny days but I have routinely photographed events where I needed ISO 3200 at f2.8 and been at 1/30s which is too slow a shutter speed to avoid subject motion blurring (for that 1/80s or faster shutter speed is required most of the time).
I no longer feel the need for f1.4 primes with the ability to shoot at ISO 3200 but that is very different from thinking for a second that a f5.6 zoom is not going to be a severe handicap for shooting in many conditions.
There is also the aspect of separating the subject from a distracting background. With f2.8 the background will be sufficiently OOF. At f5.6 this would not be the case.
When people are buying faster glass as a matter of course and you do not see the need it may be that there is no legitimate need or it may be that you need to get out more.
