After Soarer made the converter available, my Model Ms have been gathering dust as I use Model Fs as my daily drivers.
That said, I have a couple of early (heavy plate, pre-1988) heavily-modded Ms in storage, along with a couple of super-clean early 1990s black labels, so I am pretty well stocked up for life.
But, if I was going to pay over $50 + shipping for a Model M, it would have to be a very clean, very early 1391401 or 1390131, otherwise I would spend a few more bucks opt for a new Unicomp. The only real disadvantage I see is the hideous nameplate on the front.
I like the lighter-feeling springs, and the "modern conveniences" would be another plus.