SLRGear.com has tested both the DA 15 and the Tokina 12-14 (which is optically equivalent to the Pentax) and here's the conclusion:
Quote: In fact, the Tokina 12-24 handily beats the Pentax 15mm ƒ/4 on sharpness at f/5.6, and even does so slightly wide open as well. It also beats the Pentax 15mm ƒ/4 on shading (vignetting), but falls behind slightly with chromatic aberration and distortion. Overall, the Tokina 12-24mm ƒ/4 AT-X 124 AF PRO DX gives the Pentax 15mm prime a good run for the money.
Personnally, I have the DA 15 and love it! The 12-24 is a great lens, it may be a bit sharper or a bit better at certain apertures, but I find it too bulky to carry around all day.
The DA 15 is a small jewel, which exhibits excellent quality both optically and mechanically, and is small enough to be kept in a pocket just in case. When I travel light, I take my K-x with the DA 15, DA 35 and Apo-Lanthar 90. The whole kit fits in a small waist pouch, it is also surprisingly versatile (high quality wide + close focus, normal + macro, tele + close focus) and delivers pictures that never cease to amaze me.
Cheers!
Abbazz