I had the Sigma 10-20 F4.5-5.6. It was a very good lens, and can be available for pretty cheap. It was relatively sharp, and performed very well for long exposure night shooting.
For the remainder of the range - the Sigma 17-50 is very highly rated. I would actually recommend it over the Sigma 24-70, as the Sigma 24-70 has some very nervous bokeh. Alternatively, you can get the Sigma 50 F1.4, which is a brilliant prime lens that I use all the time.
http://farm9.staticflickr.com/8336/8095858455_3c5b9d0a94_o.jpg <-Full size shot on my k-x at F2.0.
At the long end, the newest Sigma 70-200 F2.8 EX DG OS HSM is the best of the Sigma 70-200 lenses, and performs very well in AF and IQ.