I for one considered the upgrade from the kit zoom (non-WR, but should be the same optically ?) to the Tamron to be worth the price for the faster aperture and better IQ. I've had this for a few days and so commenting on the the latter would be premature at this point, except maybe noting that this is capable of remarkably sharp shots, even wide open.
You'll find user reviews of both lenses (and others) on this site:
Comprehensive Pentax Lens Listings - Pentax Lens Review Database. These are also covered here, backed with lab measurements:
All Tests / Reviews (the Tamron 17-50 has been tested as the Canon version, but since this should be optically the same as the Pentax version the results in general should be useful).
Both the above review sources have nice things to say about the Tamron, which is why I was encouraged to get this particular lens: based on these it would seem to be a great value for the price, and, in fact, a very good lens even in absolute terms.