I'm not adding one... yet.
It sounds very good. The problem I see with most other compacts is that if you don't want to make a huge investment, you have to be a Sony or 4/3 user. I know the point of mirrorless is to step up from p&s, but if you want a backup camera, or a smaller body, then for the most part you're out of luck.
Then Sony has made some great cameras recently, but almost nothing with regards to lenses.
Micro 4/3 has a much smaller sensor, however, the Olympus OM-D looks really nice, I'm really tempted to rent one and play with it. And for a compact maybe the sensor isn't too small, and right now some of the older bodies are getting pretty cheap.
Nikon, I wouldn't even consider one. The 2.7x multiplier means I need a 7mm lens to get the same as the wide end of my 12-24 on a Canon body (~19-3
Canon, well, all I would need is an adapter to use my glass. I wouldn't think about using my Sigma 80-400, but to have it as a small camera that's a lot more discreet, it's not too bad of an idea. I can use my optics and have a much smaller package. Presumably I can also take it to venues where they don't allow "Professional equipment", or where a 7D with a grip would draw too much attention.
Last week I went to a local park to photograph 2 old locomotives they have on display, and I was afraid that the 7D was going to draw too much attention, so I removed the battery grip and the lens hood to look like every other teenager. I only got there to realize that it's such a popular spot for wedding shoots and my camera would have looked normal. But for situations like that, a mirrorless with a kit lens is probably all you need.
I'm definitely interested in checking it out once it comes out, maybe not buy one soon, but it's staying as a possibility.