Originally posted by RioRico Super-duper-wide lenses always distort.
That's right. As an example, consider this photo from the DA 12-24 at 12mm. Notice how the trees begin to bend toward the borders of the image:
How noticeable the distortion is depends on the subject. It's less noticeable, though still apparent, in this image, also taken at 12mm:
I wouldn't get too caught up in such issues as distortion, CA problems, or even resolution when choosing an UWA zoom lens. Get the best lens that you can afford that fits your specific needs.