I actually just "went" (but not completely) mirrorless when the EOS M got an autofocus speed upgrade and a price cut to clearance levels (I paid $350 for the body and kit lens). I am so glad I bought this camera.

I still use my T1i DSLR with "big glass" at places where I need reach (I could get a lens adapter for the EOS M, but if I'm rocking a 100-400mm lens on the camera, the smallness of the mirrorless is insignificant, and I'll generally want an optical viewfinder) or faster autofocus and I plan on buying a 70D real soon to replace the T1i, but the EOS M comes with me everywhere. And the image quality is soooo good for a go-everywhere camera (the sensor is the same size as the one in my DSLR, and even better in a few minor respects since it is basically the T4i sensor)... Image quality is leaps and bounds above the Canon S100 which was my previous attempt at such a take-everywhere solution.

Here's a shot I would never have gotten if the EOS M wasn't small enough that I can carry it everywhere (keep in mind this is just handheld using a bridge railing for stabilization, no tripod). I got this on the way to the parking lot after meeting some friends at the last Culture & Cocktails event at the San Diego Art Museum:

http://www.gmcbay.com/pic/bpark.jpg

There are a few more examples accessible via directory listing here, all from the EOS M for which I've since bought two more lenses (it still all fits in a very small Lowepro Lowepro Compact ILC Courier 70 bag even with the three lenses):

http://www.gmcbay.com/pic/

Do you mean the RX1? http://duncandavidson.com/gear/sony/rx1/

I have one, and it's pretty impressive.

For me going from a D700 to an OM-D, the biggest benefit was the much smaller size. And much of that size was in the lenses, not the bodies. The m4/3 lenses are so wonderfully small, after hauling around a 24-70/2.8 for so long. I think you'd be missing much of the benefit.

I think a lot of pros will add a crop-frame mirrorless camera to their kit when Canon release their speed booster (and a decent EF-M body, which will probably be at the same time).

The ability to use existing full-frame lenses at the same focal lengths on a mirrorless camera, and gain a stop of light-gathering ability, will be a big draw.