I had some expectations to this lens. It seems like a very useful zoom-range for a walking-around-all-day type of lens. That doesn't help when it's not able to produce sharp images. My copy didn't even get close to being sharp.
I did a comparative test with this lens and my Canon EF-S 18-55 lens (Canon kit lens). The Canon kit lens was not only better, it was so much better I could hardly believe it. The Canon kit lens is of course itself not a very high quality lens.
I might have been "unlucky" to get a bad copy, but then that does seem to happen a lot with Sigma lenses, reading other people's experiences. If you're planning to buy this lens, you should ideally try it before buying, or at least make sure you can return it and get your money back. If you get a copy as bad as I did, you will not be happy with it. Ever.
As for me, this was my first Sigma lens, and probably also the last. At least for quite some time...