Hi Simon, I have the Tamron 18-250 and love it.. prior to this I had both the Sigma 70-300 and the Sigma 18-200, both of which I found to be rather poor unless stopped down a lot.
The Tamron has consistently blown me away with it's clarity and contrast, and rarely leaves my K10D now. It is also a great Macro lens, although at 250mm is only good without flash in daylight.
This example of a peach blossom was shot at 250mm and F16.
The Sigma 10-20 EX is also in my bag, and this too is an exceptional lens, and I use it a lot more than any prime I have. In fact I am close to reducing my lens arsenal down, and the first ones to go will be the SMC 50mm 1.7, and the SMC 50mm 1.4 FA. Frankly, the 10-20 IMHO is a better option.
This shot of the Sydney Harbour Bridge was shot with the Sigma 10-20, 10mm at F11. As I was almost under the bridge, there is an interesting amount of distortion and this has not been adjusted in any way.
Phil