I got my M today. Just a few quick impressions, so I'll piggyback on this thread, as I don't really have anything very useful to add:
The first thing I noticed is how incredibly dinky it looks - but it feels really nice and solid. The flash, though, actually caused me a fit of the giggles. It seems ridiculously small if you've been fiddling around with a 600EX
I can see why people are unhappy with the AF, it's not exactly speedy. Then again, I've been testing indoors in fairly low light conditions, so maybe it's better outdoors/in daylight. For me personally, it's not much of an issue anyway, but I'm guessing it would annoy folks who like to take action/wildlife shots.
I had a few befuddled moments with the menus, but it's not my first Canon, and I didn't take long to figure them out and feel at home in them.
It's perhaps worth noting that you sort of have to adjust to how it's best held and operated - it felt a bit awkward at first, but I soon got used to it.
I basically bought it because it was so cheap, and wasn't entirely sure I'd keep it, but after playing around with it for a bit I think I'm going to like it, and it will suit my purposes just fine. (I have a 7D with what feels like several tons of lenses, which I really love; I also have a Digital Ixus, but I'm just not happy with that anymore since I got the 7D, nor do I enjoy taking snaps with my phone. So I'm hoping I'll get some joy out of the M on those occasions where it's just not worth lugging a backbreaking amount of gear around all day for the few shots I get around to taking when hanging out with friends who aren't avid shooters.)