I have several LTM-M adapter rings- a few Leitz ones, a couple of Cosina/Voigtlander ones, and one un-marked knock-off. The Leitz and Voigtlander adapters work just fine with all my LTM lenses and on the each of the dozen or so M bodies I've tried them on. But the no-name knock-off (which I got as part of a body/lens kit off of eBay) is useless. It will mount lenses, but I have never been able to make it lock onto any M camera body. Close inspection reveals that the locking slot was machined ever so slightly out of place. The lens mount button never clicks.
I expect that somebody could make a cheap knock off adapter ring which would work just fine. But the tolerances on these things are very fine, and they have to be really precise. Considering that you can sometimes get old Leitz ones for $40-$50, and the Voigtlander ones sell new for around $55, I think it makes sense to pony up the extra money at the outset. If you spring for a $20 knock-off and it works as well as the one I have, you'll spend the extra for a good one later. Might as well spend a little more now and save money in the long run.