From the rough comparisons, the version II is definitely a little sharper than the original version I. Pentax tweaked this lens and got it to perform better.
The most interesting thing about the lenses was the fact that despite the camera exposure being the same on the K20D (and referenced to a hand held meter), version I tended to produce images that were a little underexposed. This was also evident when other auto exposure modes were used.
When the version II was used on the K20D, exposures were consistent and worked the same on all the auto exposure modes.
The newer lens shows better contrast and without PP gives "snappier" images. Both lenses still showed some barrel distortion at the wide end but nothing to worry. The good thing is the version II appears to vignette less compared to the version I at 18mm.
I would say that version II is definitely the better of the two lenses but it still remains a starter lens. Certainly a good lens to keep as a cheap wide angle.