Not to poo-poo on the DA15, but I "think" the reason it "controls the mind" is because it is the best lens in that range. It is a beauty for sure, but if it is the wrong focal length... that kind of trumps it being superb quality. An extreme example being I need 300mm length, but the 50-135 is sharper... wouldn't make sense to buy a sharper lens I don't need. But in your case, a 15mm can be cropped to 21mm.
When you had the 18-135, did you do most your shooting at 18? Or was more of it above 18? Personally, I need a 24-28mm, but Pentax does not make a new 28mm
Edit: Also, with the 21mm, I think it gets a bad rep for trying to do so much in a little package. It has to compete with the awesome zoom lenses around that focal length (24-xx). It is small.... but slow... and distorts... and an odd focal length...It is like loving an ugly child. The 15mm is sexy, has less great lenses to compete against...