15Ltd

The 15 is one of the best reasons for me to stay with Pentax, it is better than you may think. I was recently asking myself the same questions regarding shrpness at 15mm between the 12-24, 10-20 and the 15. If you want, I can send you the raw files from a very qad comparison I did this week: comparison shots of 12-24, 10-20 and 15mm at f5.6

I find it soft at the edges, as others have said, wide open - though it doesn't often matter to me. I have found there is some risk of vignetting with filters. But I forgive all that because the 15 is a versatile lens and capable of great shots.

Since buying a 15mm Ltd I hardly use the Sigma 10-20, largely because of it's size, but I also find the 15mm sharper, especially at the edges. There's also less distortion at the edges, very noticeably so when using it for street photography. It's probably my favourite lens, the images from it just seem to have a look that the Sigma doesn't quite match.