Been going back and forth with lenses for a while now, and finally have my bag full. It's filled mainly with Sigma, but when this Tamron came out, I just couldn't resist the 2.8 through the zoom range and the VC (vibration compensation). So how is it? Well, I love it. The other reviewer here mentioned it's a little loud, and that's true. But that doesn't bother me one bit. It's very, very sharp, generally focuses well and quickly, even in low light, and has accurate color representation. At 17mm 2.8 it's a LITTLE softer, but not much, and once you stop down it gets tack sharp, which is true for most lenses anyway.
As an end note, something needs to be said for Tamron's vibration compensation technology. I recently sold my 18-55 VR Nikon kit lens because I got this Tamron, and the VC works at LEAST as well as the VR does, if not measurably better. Yes, there is a jump when you depress the shutter button halfway, as the motor starts up, but it's something that you get used to extremely quickly (or at least I did), and once that's over, the image is still as a quiet summer lake. The VC works VERY well.
Overall, I'd say the build quality of this lens is very good, and it's definitely a keeper.
The one con I'll personally give it is that the macro capability leaves something to be desired. This isn't billed as a macro lens, so I can't really knock it, but my Sigma 17-70 w/macro smokes it.
Other than that, great lens, especially if you can't afford the Nikon lenses.