I have the Sigma 10-20 and it is the best landscape lens I have. If you prowl around on Flickr too, you'll see that a large number of the really excellent landscape photogs use this lens.
I'm just waiting now for DxO Optics to come out with a correction set for the K10D and the Sigma 10-20 combination. Post-processing the pics through that software would put the icing on the cake, so to speak
I also have the DA*16-50 SDM, and comparing the two lenses at the 16mm mark, the Pentax is a fair bit sharper at all apertures, edge to edge. But you get what you pay for, and with the Sigma you get the extra 6mm of wide angle. It's not really a fair comparison. I don't have the Pentax 12-24 to compare, unfortunately.
My bottom line advice - get the Sigma 10-20, and have it fitted on the camera when walking in National Parks etc, also indoors. You won't regret it.