Sony NEX cameras produce great images. They are not better than Nikon but generally equal. The big difference is the system. Sony simply has far to go to match the Nikon systems.
Lenses - Nikon major advantage
Speedlights - Nikon advantage (NEX-7 will not trigger a wireless remote) like the D7000/D600/D700, etc. Say goodbye to Nikon ClS if you move to Sony - something I was unwilling to do.
Camera Shake - Nikon advantage. It is hard for me to hold the NEX-7 still enough to avoid camera shake. Try it before you buy it. Take a SD card and shoot at the camera store and then compare to a full size SLR.
Focus speed and accuracy - Nikon advantage in nearly all bodies above the entry level.
Resale value - Nikon advantage, especially in lenses.
In-camera processing and special effects - Sony advantage. Love the pano features, HDR painting, etc.
Size/weight/carrying - Sony advantage. It is light and small so it goes with me more often than the D800.
Cropping - D800 advantage. With 36mp it gives flexibility to crop and can stretch utility of a zoom by cropping to get a closer look.
In the interest of full disclosure, I have both the NEX-7 and the Nikon D800.