The Pentax 18-250 is my most used outdoor lens. I keep coming back to it, because it is so versatile. I did a comparison of my 18-250 and the DA* 50-135 on both the k20d and k10d and the only way you can see a difference is at 100% crop. Here are the pics:
k10d and k20d DA 18-250 and DA* 50-135 - a set on Flickr
MZ stands for the 18-250 and DAstar is the DA*
for those of you that don't want to go through the whole set, here is a
100% crop at 100mm for both the DA* 50-135 and the 18-250. Guess which is which:
(I always chuckle when I read some posters that claim that the Pentax 18-250 must be "soft" because it is a "megazoom." I guarantee that you cant tell the difference without going to extreme measures such as this.)
Here is one of the full images that the above was cropped from, so you can see how extreme the crop is: