I've used digital SLRs and digicams since they came out, and have owned a D1x, a D2x and two Kodak full-frames, along with several pocket digitals; and my son owned a Canon 1Ds. I no longer pay much attention to megapixel claims, because so much depends on the chip itself and the firmware and software behind it. One problem that is constantly encountered in Internet examinations of these issues is simple lying -- the people who claim to have knowledge of various cameras have never used them, and after a while, you can tell that. Many Canon freaks have never really used a Nikon, and vice-versa. You simply CANNOT tell how a system will work until you have it and have tried it. There are also practical aspects: Canon keeps going for more megapixels, when, in truth, you really can't tell the difference now between a 16mp Canon 1DsII and a 12mp Nikon D2x when printed as a double-truck in the best-quality magazines, because the printing on high-speed presses -- even the best presses -- is so much less good than what either camera can do, that the differences are obliterated. There are also distinct benefits to a Nikon-sized sensor; and there are distinct benefits to a Canon-sized sensor. (I refuse to use the term Full Fame here because it no longer makes any sense, and never did make much.) My feeling is, and I certainly could be wrong, is that the M8 will match both Nikon and Canon for almost any practical purpose, although it might not match a 1DsII in resolution when the 1DsII is mounted with Leica R lenses longer than 35mm. With any lenses shorter than 35mm, the 1DsIII is going to have corner problems, but that's another issue.
The critical aspect of the M8 is its size and handiness. As I write this, I have a D2x on my desk along with an M7. The D2x (which is a great camera) is about twice as tall as the M7; iun fact, it is TALLER than the M7 is WIDE by, I would say (just eye-balling) about an inch. It's also wider by at least an inch, and, on the hand-grip portion of the D2x, thicker by an inch and a half -- it is more than twice as thick as the M7, and it weighs a ton. I keep an RRS backet on the D2x, and that makes it even wider and taller and heavier. The lens on the D2x, an 18-200 zoom, is, at its shortest length, almost as long as the M7 body, and when I heft it, the lens alone feels almost as heavy as an M7 with a 35 Summilux ASPH attached.
The 1DSII is significantly bigger than the D2x.
The point here is, Ms and SLRs are radically different cameras, and are used differently. I doubt that many workaday photojournalists will get M8s, because SLRs are more flexible, and flexibility is what workaday PJs need more than anything. The M8s will go mostly to the art crowd; and the art crowd is actually quite large.
If I actually knew how to post photographs on this forum, I'd take pictures of the two systems and post them...but just sitting here looking at them, comparisons between a top end SLR and an M7 are like comparing a roadster to a hardcore SUV. Which is best depends on what you're planning to do.
JC