I have before, but not your classic idea of a neck knife; I carried a Folder like that, and found it great.
For a couple years, I carried my Case Peanut around my neck in a custom kydex sheath I made for it. It was a lot faster than digging in a pocket for it, and a great bonus if sitting, I could get to it SO much easier then. I got out of doing that when I was doing a lot of machine and mechanics work; something hanging around your neck is suicidal then.
Lately with my light around my neck 24/7 (except time at the lathe in the shop) I have considered carrying the ‘nut that way again, but a lot of the convenience of the light hanging there is being able to turn it on and leave it hanging there to shine down… I tried it for a few days a couple months ago, and the knife hanging there either blocks it completely or adds a shadow in a bad/funny place. Might try it again for a few days and see how it works if I space them out vertically.
I had also tried a small fixed blade there at times, but the shear size of it(even in a minimal or skeletonized 5″ or less oal knife) was too big, it swings around a lot, and hangs down into my way when bending over working etc. The solution is to put it under my shirt or jacket, but then its a pain to get at to draw the knife, at that point I might as well belt or pocket carry it.
Its actually funny, one of my takes from the pass around box this week, is a small 6″ OAL dagger neck knife… I don’t wear fixed blade neck knives as said above… but I still kept this little knife… Its just such a nice size and setup for the knife style, it looked fun to have and try to wear. Will see now it goes I guess.
The peanut in its Kydex;
I have just now, while thinking this out, taken the sheath off of the para-cord, and put it on a short dog tag chain, and a S-biner. I can then hang it on the cord I use for my light in the same way that I hang the light.
Also shortened up the chain on the new knife to the same setup as the Peanut. Will see how it works around the house some time soon.