I have this lens, I find it better than the 18-105 and 18-200 both of which I have as well.
Its extremely sharp and the wider 16mm end is especially useful for landscape and general photography. You will not regret buying it, although bit expensive compared to the 18-105 or 18-70 or 18-200. Even at 85mm fully acceptable in my opinion.
it's a great lens, one of the best standard DX zooms. but then again, do u shoot at such focal lengths often? if u tend to shoot in the tele range, u wun enjoy using this lens, coz it will leave u craving for a tele lens since it only reaches up to 85mm.
Used to own this lens, in terms of performance the IQ is great
Some people may prefer the 18-200mm because of the additional tele range, but many people and reviews have concluded that its IQ is nowhere as good when compared to the 16-85mm
Plus the additional 2mm wide is very handy if you shoot wides quite often
I have used both.
If you can get a sharp copy of the tamron, go for it.
But, stick with the nikon if you don't want to the hassle of finding a good copy (they are always spot on).
I tried three tamron 17-50 lenses at OP and found only one sharp when I bought mine.
The other two focusing were off.