I have the 18-200. It is a perfectly good walkaround or travel zoom, good contrast, colour and acceptable sharpness (if stopped down a bit). It could be a bit sharper at 200mm and wide open, but it's general performance isn't too bad.
The 18-250 is universally said to be better, but if you can get an 18-200 for a good price go for it.
Both lenses are reviewed in detail here:
Tamron Lens: Zooms - SLRgear.com!
These reviews are somewhat old though and I suspect Tamron have probably introduced a few tweaks and improvements since these reviews were made, since my copy of the lens doesn't seem to exhibit the softness pattern shown in the SLRGear tests.
Oh and CameraLabs did a big review of the lens in 2006:
Tamron AF18-200mm F3.5-6.3 XR Di-II LD IF DSLR lens full review Cameralabs introduction