Jens,
There is no problem at all with the adaptation to the K10D. I use my 645 lenses as needed (mostly the long telephoto 600mm ED IF) and it mounts beautifully with the adapter... which is not cheap (about 250 from Pentax).
The lenses as you know are larger than there 35mm counterparts, and even moreso with the DA versions. But, they do work, do use the sweet spot and do save you the expense of a redundant lens. OF course, if you are old school, the manual operation should not be a problem.
Stephen