After using the NEX-5N for just a bit over a week, I have one word to describe the image quality: wonderful. Really, the improvement from the original NEX-5 is huge. In low ISO, shadow noise is mostly gone and the image looks like it comes of a Canon 1Ds II. The biggest improvement is in the high ISO area. I compared the noise level to my 1D III at ISO 1600, and they are roughly equivalent, with a slight edge to the 1D III, which retains more details. But, the 1D III has a 1.3x crop factor with only 10 MP, and thus the pixel size is much larger, whereas the NEX-5N is 1.5x with 16MP. This is an incredible achievement.
I don't think the NEX-7 will be anywhere as good as the 5N in terms of noise performance. While I will miss the handling and built-in EVF of the 7, image quality matters more to me. When the external EVF goes down in price, I will pick one up for the 5N. One of the good things about external EVF is that it can be used on the next model (hopefully) and can be sold separately if I later upgrade to one with built-in EVF.
In conclusion, I have no complains about the image quality of the NEX-5N. I am VERY glad to have bought it. Now, why didn't Sony just use this wonderful sensor on the NEX-7, or, make a model with NEX-7 body with the NEX-5N sensor and priced between the two? It will be a perfectly balanced camera in terms of handling and image quality.
Cranes -- NEX-5N & Leica-R Elmarit 28mm f2.8, ISO 400.