I have a Nikon D800 and a D3, and also a large hybrid Panasonic/Olympus (lenses only for the Oly) system. The *only* reason for buying a mirrorless system IMHO is compactness, both in body and lenses. And it doesn't take much of an upward size change to make a serious difference in weight and compactness. For example, I have both the new Panasonic GH3 and the older GH2, and while, when you look at the specs, the GH3 seems only insignificantly larger, is IS larger and that small increase in size has taken it from "compact" into DSLR territory. It still uses the small lenses, but it's somewhat lost its grip on compactness. The fact is, for almost all *critical* applications, the Nikon system is better. There are good reasons for size and weight: you get more technology, you get better ergonomics, etc.
BUT: I use the Panasonic for almost all informal and street people-shooting, and also when I travel for non-photogaphic reasons. My best travel kit is a GX1 and two zooms, the Panny 12-35 f2.8 and the 35-100 f2.8. These are big lenses *for the system,* but they cover the equivalent of 24-70 and 70-200, and, compared to other f2.8 zoom lenses for larger sensors, are very compact.
I think people obsess too much about sensors. Most mirrorless cameras are made to be hand-held, and if you're going to hand-hold a camera, the quality of the photo will depend much more on your hand-holding ability than in the differences between sensor sizes. And if you're NOT going to hand hold it -- if you're going to use a tripod -- then why are you looking for mirrorless at all? You can get better cameras with better resolution and better lens systems by going to a DSLR, and since you're carrying a tripod around, what difference will a few ounces in camera- and lens-size make? And, the lower end DSLRs won't cost much more, if any more at all.
I chose the Panny system over the NEX simply because compactness was the most important consideration, and since a camera has to have lenses, I wanted compact lenses, too. Generally speaking, the NEX system doesn't have them -- the lenses are the same size as the lenses for a Nikon APS-C. If you're basically a prime shooter, your mileage may vary. (But I don't know why people with less-that-maximal-quality cameras are prime shooters, either. The difference between a prime and a high-quality zoom is almost non-existent. If I used a 35mm Zeiss prime against my 35-70 zoom @ 35mm, I suspect the Zeiss might come off better in technical studies...unless the good frame for your shot was, say 50mm. Then, you shoot with your 35 and crop; with the zoom, you crop with the lens and use the whole sensor. And so on...)
M4/3 IMHO beats the other systems mostly on system size and flexibility -- the choice of lenses. NEX falters on lens size and lens availability, although the quality is excellent if the lens is good. The other contenders don't have the system size for the time being, or the lens-included compactness. Again, for serious photographers, I think compactness, and the advantages that confers, is the only reason for going to mirrorless.