The nikon beats the tamron hands-down from 71mm to 200mm
Whereas the tamron blows away the nikon at 17mm
You essentially are comparing a general usage lens to a more specific fast lens.
Tamron is probably better especially if you need a fast lens with in the focal range.
If you need anything low light with the proper depth of field then the Tamron at 2.8-4.0 aperture will clearly beat the Nikon.
But if you need a long lens then it's the Nikon.
It all comes down to what your needs are.
No comprison here, focal range is way too different and the Tamron is much faster.
Also the Tamron is sharper and the Nikon 18-200mm is a superzoom which means it isnt too sharp.
Its like comparing a minivan to a Honda Civic.
I have to agree with the general consensus here in that you're trying to compare apples to kiwi's...not really the same thing. If you're considering a purchase, my suggestion would be to ask yourself -why- you're considering both/either of these lenses. If you're looking for something that will do decent scenic landscape shots, the Tamron would be the hands down winner. If you need something that's going to give you an edge with low light work, I'd go for the Tamron in a heart beat. If you need fairly wide angle shots that are going to be sharp and require less post-processing, I'll take the Tamron any day. On the other hand, if you really feel you need to rack the zoom out to 200mm, then...well...the Tamron only goes out to 70. Should be a no-brainer there.
I will say that I do own a Tamron 18-200mm and that it does in fact have it's uses as a "walking around" lens. For example, this is how I might compare the usage; if I were going to the zoo with my family for a simple afternoon of walking around to see all the critters, I might take my 18-200mm. It's VERY lightweight and it would save me having to drag around a lot of extra stuff in my camera bag. It's certainly nice for a few snapshots and on my D90, it's a hell of a lot better than my stupid little Sony 16.1mp compact. Conversely, if I were going to that same zoo by myself with the specific intent to get some good pics of some specific critters, say for an exhibition, then I'd take a couple of my better lenses instead and leave the 18-200 at home. Broadly speaking, if I'm looking to shoot pictures that I plan to print and frame, I use the better lenses. If I'm just porkin' about town or scoping something out as a potential photography subject, I'll take the 18-200. That's not to say that you can't get ANY decent images with the super-zoom, but you have to understand that those lenses in general do have their limitations. I don't really know how well the Nikon version compares, but these super-zooms can be quirky beasts to say the least...they suffer from a bad degree of distortion on the short end and tend to be rather soft on the long end. In fact, my Tamron 70-300mm does better at 300mm than the 18-200 does at 200mm.
So between those 2 lenses, which is better? Depends on what you need the lens for.