I got this lens in a kit w/ a K20D body (excellent overall camera). After looking at the positive reviews here I had to do a double-take, to see if we were talking about the same lens (we were) and the next question was, if, as some reviewers noted, this is an improvement over a previous model, I don't think I'd want to use the original.
Don't get me wrong, the lens is not bad. Construction quality is good and it works well with the K20D. The biggest problem I have with it is at short focal distances 18-24mm I can clearly see (and I am not a pro by any stretch of the definition) the problems with image quality:
1) Distortion (images start looking fish-eyed)
2) Vignetting
3) Chromatic aberration
Now, there are two pieces of good news: (1) these effects almost completely disappear at 24mm and up (2) at shorter focal distances raw processing software can deal with these problems to some extent (I use Bibble, YMMV). At 24+mm this lense is EXCELLENT and overall a good value. So the drawback is that now any time I take a picture with it I have to weigh the advantage of a wider angle vs. the guaranteed distortion and other problems I will have to deal with in post-processing.