Pentax 6x 7 lenses are great.
I have mainly used them for a (KAF16803 sensor 36mm square) CCD imaging so I don't see chromatic aberration much and haven't tried it on a digital camera.
The ones that are best are:
1. 75mm F4.5
2. 165 f2.8 (mine is good wide open)
3. 300mm F4 EDIF also the 645 300mm F4 EDIF
http://www.pbase.com/gregbradley/image/167800780/large 300mm
http://www.pbase.com/gregbradley/image/167787138/large 300mm
There are example images on my website for the other lenses.
These lenses apart from the EDIF versions are quite cheap as well. The 75mm cost me under $100, the 165 under $200.
Greg