This is a neat lens! Great for walk-about, casual-shooting, and travel. Build quality is on par with other Tammy's I have owned. I like Tamron's build quality better than the non-L Canons. The double-telescoping zoom extension is a little unnerving as it comes out so far, and looks a little fragile, so one must definitely must use the zoom lock when walking about.
The 18-250mm has very good image quality at 18mm. I was worried about vignetting, but was really not evident at all when I tested it on the daytime sky. As expected with such a wide zoom range, it's seems a trite soft at maximum magnification compared to my 70-200L, although nothing a little unsharp mask can't take care of. Plus, one has to remember remember that on an APS-sized chip, you're shooting at 400mm. High shutter speeds or a tripod are in order!
The autofocus can hunt in low light and is not the fastest even in good light.
Overall, a pleasant surprise. If you need a compact walk-about, hiking, or casual shooting lens, this is the one. I great third or fourth lens for the experienced shooter, or a versatile starter lens for the beginner who will be shooting in mostly in daylight.