One hopes that Nick's tongue is firmly in his cheek; otherwise his head is up his ....
Indeed, different tools for different purposes. Can you imagine quick action street work with a 645D? Better be quick with that mirror lock-up and tripod. And better have some good light with not much high ISO capability and slow lenses. Oh, did I mention lenses? Seems he overlooked this small reason many buy into the M system, perhaps to a greater degree than even the small camera form and RF viewing. (BTW, I've owned the film version of the Pentax 645, as well as film Ms...complementary, not comparable.)
Let's also not forget print sizes. For me, print quality is all that matters. For prints sizes under 24 inches, one can't tell much between the M9, 645D or the Hassy/Phase systems. See, for example,
here or
here. [Note also various early problems with the 645D.] In fact, this is one reason why I have 2 M8.2s rather than 2 M9s (which I could afford); don't print large enough to see the difference.
Buyer's remorse is another oversimplification. Some buy an M9 (or M8) as their first rangefinder and just don't get along with it. Not everyone's cup of tea. Some use an M for a while, and then age starts messing with their eyesight. If one can't see well enough to manually focus, then a key advantage and reason for owning an M becomes a hindrance. Some realize too late that an M is not an all-around tool (for most people)...while some manage long lenses, macro (Viso) or sports/action work, these are not the M strengths. Or, perhaps they didn't admit to themselves that, after relying on their DSLRs, they were too lazy, or not knowledgeable enough, to learn to effectively use a primarily manual tool.
A good photographer will make do with about any tools. But, if they have the resources, they will own the best tool(s) for their own needs and preferences (which by then are well defined). And in the digital world, the required tools go far beyond the camera and lenses...one better have the right software, hardware, papers, inks, profiles, etc -- and know how to use them effectively, both separately and together -- to make the best prints. The chain and its weakest link and all that...
This whole mindset of my camera is better than your camera...not just simplistic, but quite childish and tiring.
Jeff