Originally Posted by mhv
The main problem with the M42 lens mount was that it came out rather earlier than most; although it had a wider throat and shorter register than the then de-facto standard 35mm SLR, the Exakta, later cameras tend to have larger throat diameters or shorter register, or more than often, both, so that they could offer adapters to fill in the space in order to take advantage of the big pool of M42 lenses out there.
It also means that the reverse situation, fitting bayonet mount lenses to a M42 camera would be extremely difficult, if at all possible. Sure you might be able to get an adapter custom-made (more than likely at tremendous cost) but if the register is not matched, you are effectively using the lens on an extension tube making even general photography impossible.
If you like to use a Nikon (or any other brand) lens you cannot realistically do that without a Nikon camera. But then, there are still no shortage of very viable M42 lenses on the market. Those new ones by Voigtlander are superb, Russian ones are great value for money too. The "original" lenses supplied to dozens of M42 cameras over the years also makes a good resource, let alone the ones by third-party makers: apart from the more modern ones, do not forget houses such as Albert Schacht, Steinheil, Schneider, Rodenstock, Angenieux, Enna, Piesker, Tewe, Novoflex....
For myself, my requirements for M42 lenses are very well satisfied by "original" lenses supplied to Praktica over the years, mainly by Carl Zeiss and Hugo Meyer, along with a few exoticas by Angenieux, Steinheil, etc.