Check this reviews from CameraLabs.com, they are very happy with Pentax DA 18-250:
"DA 18-250mm ... despite its super-zoom range, actually performs very well across the frame at the test focal length" (K20D Review,
Pentax K20D review Cameralabs outdoor results)
"... you can't sharpen a lens which is soft in the corners, and in this respect the Pentax DA 18-250mm certainly has an advantage over the other two lens and body combinations tested here ..." (Olympus E520 Review,
Olympus EVOLT E-520 review Cameralabs outdoor results). There are some comparison images as well.
The bad thing about this lens - creepy zoom...