Shun Cheung , Apr 06, 2009; 11:51 a.m.
Personally I would not buy any AF lens that is slower than f5.6 in any part of its zoom range. Nikon's auto focus becomes unreliable when the lens is slower than f5.6.
I have neither one of those lenses, but I have used the Nikon 18-200 briefly. While you certinaly don't expect great image quality from a 11x zoom, I think it is optically fine for a 11x zoom.
Another issue with the Tamron is that it is almost just as expensive as the Canon and Nikon equivalents. However, the Tamron's value is unlikely to hold up in the used market as well as camera brand lenses. Typically one buys 3rd-party lenses mainly because they are much cheaper up front.