I have that lens - it is a decent performer as a walkabout and travel lens, and I got it at a good price sometime ago. If the price is right, go for it.
But if I was buying a similar lens again I'd go for the 18-250, which has a reputation as a better performer than the 18-200 and gives you a useful degree of extra reach. A good review of it is here:
Tamron AF 18-250mm f/3.5-6.3 Di II LD Aspherical [IF] macro (Nikon) - Review / Lab Test Report
Reviews of both lenses are to be found here:
PentaxForums.com Third-Party Pentax Lens Review Database - Zoom Lenses
Reviews and tests of both Tamron's are here as well:
Tamron Lens: Zooms - SLRgear.com!