I've got a Nikon F to Leica L39 adaptor, made by Voigtlander so I can mount my 15mm Heliar SL on my Leica.
As rxmd says, it is indeed rather impractical to use. You have to scale focus, so it is only realistically worthwhile with ultra wides. With the Voigtlander SL, it is fairly comfortable to use because the lens projects so little from the body. In my case, at the time I got the lens the SL was much cheaper than the L39 Heliar, and I wasn't sure if I'd end up keeping a Leica system, so it made sense to get the lens and the adaptor.
With any other Nikon lens (maybe not the 21mm Nikkor-O, which is similar in form to the Voigtlander SL? But then if you own one of those, a black paint F to mount it on is going to be nothing..), the lens is really out of proportion with the body, plus you need to get an external finder, etc. I would say the value for money is incredibly low, I paid I think 60,000 yen for mine, about £30 then, and I wouldn't pay more, especially when good Nikon bodies are so cheap these days.