Secondly, its a very versatile system. With two companies making "native" AF lenses for it, plus the 1000's of lenses that can be used via adapters, it has the largest potential lens choice of any photographic format. And while using these alternative and legacy lenses is now somewhat easier on the latest NEX cameras, it is still a pleasure for me to use an m-mount or Nikon D lens on my Pens or GX1.
Thirdly, within certain limits, the sensor manufacturing half of the "coalition" Panasonic, have come up with a quality product. We all know the limitations that exist at high ISO's and that we have to take care with subjects that have a very wide dynamic range, but certainly for the majority of my own use, I have few problems with either of them. Plus there is a special quality that these sensors have when used at their base ISO's in good light. The fairly weak anti-aliasing / low pass filters allow a good deal of detail to be recorded. The images are sharp with good contrast and excellent colour rendition. I've always described m4/3 colour as "glossy", and to me that best sums it up.