I have a NEX-5.. it's currently my most used camera because I can carry it anywhere.. with the pancake 16mm it fits in my pocket as well..
the sensor on the nex is the best part.. very high quality, low noise at high ISO and only a 1.5 crop factor vs. 2.0 on the m4/3 cameras.. I'd compare it's sensor performance to any entry-mid level APS-C SLR..
yes the weakest factor is the Sony lens line up right now but like the m4/3 cameras you can pretty much adapter any lens to the camera.. I currently have a bunch of Zeiss Leica rangefinder and old classic lenses on it and the performance is stellar.