Had 2 Sigma 10-20's and returned both.
Tried a 12-24 and it was in a different league, superb IQ and excellent build quality, one of the best Pentax zooms.
It even has less distortion and is sharper than the prime DA14 at any aperture. It's the definitive WA zoom for Pentax, worth the extra over the Sigma any day.
I'm also very fond of the DA 10-17 as I often prefer no perspective stretch to the fattening effect that you get with rectilinear wide angles.
Examples:
Fisheye at 10mm
Corrected to rectilinear format:
It's even still evident at 17mm
Corrected to rectilinear, the guy on the right is still a bit fatter than in reality!
You have to be careful with both types of lens of course, which is why you'll probably need both! At least with the fish eye, you can defish, but theIQ is not as good as compared to a properly corrected rectilinear wide angle lens such as the DA 12-24.