Originally posted by Adam They are cheaper to make. As a trade-off, you lose infinity focus.
If you plan to make extensive use of M42 lenses, it's worth investing in the Pentax adapter, IMO.
A couple of points worth noting:
(1) Some vintage lenses, such as the Helios 44-2, Industar-50-2 and others, don't cover the electrical contacts on the camera's mount, nor do they cover the cut-out areas of the flush-fitting adapters (such as the official Pentax M42 adapter). That allows dust and dirt onto the contacts, but - more importantly - can lead to light leaks and dust ingress through the cut-outs. If I'm using a lens where this is a problem, I'll often choose to use a good quality
Kipon flanged adapter (so long as I don't need long-distance focusing).
(2) The original Pentax M42-to-K adapter is great quality, but becoming harder to find, and isn't usually cheap. I have one, but I actually prefer the
Kipon version. It's excellent quality, nicely finished, and has an inner flange that protects the SDM contacts inside the camera. This is useful if you're fitting a lens with a long and sharp-ended aperture pin, which can otherwise scratch those SDM contacts when fitting and removing the lens.
My recommendation would be to buy both a flush-fitting AND a flanged adapter (both good quality models such as Kipon)...