I use a 5DMKIIII for all my work but have a little NEX 5N similar to the one your looking at.
The sensor is fantastic, amazing detail, very sharp, lots of DR and a great camera to use. I dont think your mad they are smaller, much much more portable, the image quality will be similar to what your used to with the 5DC although low light may not be quite as good.
But the camera is a compromise, but IMO a good one. Although you cannot replace your DSLR with a mirrorless for everything as of yet unless its a fuji I like the X100s and Xpro1 but they are quite expensive, even those dont track particularly well. Bang for buck nothing beats the sonys really. I have taken some great pics with mine, for little trips daily camera to have around the DSLR is a bit overkill and the Nex does a great job. It doesn't cut it for my work tho and the 5DMKIII blows it away but again its 5-6x the size without a lens.
Only problem with the sony kit is that there is a lack of lenses but hopefully in time.