I also thought about choosing a lens for a wide angle after buying my k-1. For K-5 and K-3, I had a sigma of 10-20mm.
I checked my photos taken with sigma and found that most of the photo is taken at 10mm, which match to 15mm in full format.
Therefore, I decided that it is more right to buy the Irix 15mm and the D-FA-24-70mm, which is much more versatile, which I did.
I am very satisfied with the Irix, it’s very good, sharp lens, it does not have autofocus, but for the landscape and 15mm it does not matter.
For most my shots I set it on at infinity and set the aperture more than f5.6 to get every things in focus.
Build quality is superb, focus is smooth with clicks in at infinity. In addition, its front lens is not so convex, which allows the use of conventional screw-on filters.
I will give some examples of landscapes captured by this lens: