Cajunphotog, for what it's worth, my wife uses a Tamron 18-200mm Di II on the Nikon D300s she uses at work, where she takes many low-light shots, which includes macro work out to objects several blocks away. (crime/death scenes) This is equipment issued to her by a government entity.
Based on her satisfaction with her lens, I obtained a pre-owned Tamron 18-200mm Di II for Canons, and have very mixed feelings about its low-light performance. It needs much assistance from a flashlight for autofocusing, in my experience, lest the autofocus endlessly hunt. My wife tends to mostly use manual focus, for which she uses a flashlight, anyway. She is in no hurry, of course, as dead people are not going anywhere. I am also a public servant, with the newly added title of photographer, but am a first responder, and must often shoot photos of things moving about in low light, so I appreciate autofocus. I get better autofocusing results from Canon 18-55 kit lenses, than the Tamron, in low light! (They share a maximum aperture of 3.5 with the Tamrons under discussion.) One kit lens is the older, non-IS, and I just recently obtained a pre-owned one with IS. Notably, the Crime Scene Units working for the same agency as I are issued Canon 50D cameras, and in the field, they all seem to be content with the 18-55mm kit lens with IS, though they use a tripod most of the time, and keep the IS shut off.
To be clear, I use the 18-55 kit lenses with no filters, which lets more light into the camera, all else being equal. I generally do not need any type of filtering at night, and these lenses are cheap enough for me not to weep too much if I scratch a lens. I do have a UV filter on the Tamron, and it would probably perform better in low light if I removed the filter under such conditions. Of course, I am not advocating the kit lenses as being better, overall, than, a super-zoom, but within their limited zoom range, are not bad at all.
In daylight, however, on the hobby end of things, I really do like my Tamron, and have considered upgrading to the 18-270mm with VC for the times I want to carry just the one camera, and no extra lenses.