......Does it work for critically accurate focusing?
The thing that you have to keep in mind with focus peaking is that it shows the areas of highest contrast in the image, which are usually, but not always, the areas that are the most in focus.
It works best at wide apertures with shallower depth of field, but I find that between focus peaking and magnified live view, I can focus at my shooting aperture, which is another advantage because it eliminates any lens focus shift issues and you see the depth of field at your shooting aperture. For example, when taking a photo of a person at reasonably close distance wide open, the focus peaking will outline the dark part of the pupil or the eyelashes, which is perfect.
I find focus peaking least helpful outdoors in bright sunlight when there are many areas of the image with extremely high contrast. In those situations, one push of the button gives you eye level magnified live view for fine tuning. Push the button a second time and it zooms in even more. To avoid the issues involved with focusing in the center of the frame and then recomposing, you can push on the scroll wheel to move the zoomed in portion of the image around the frame.
So, after using the NEX for a while I find that between focus peaking and fine tuning with magnified live view I can pretty much nail focus in any situation. My percentage of critically focused shots is much higher than with AF or MF with my D300 or K5.
Also, my percentage of properly exposed shots on the first try is up because you can have a live histogram in the corner of the viewfinder and change exposure compensation and ISO to get the correct exposure and shutter speed without taking the camera from your eye. At this point, I feel kind of handicapped when I go back to the OVF of my D300 or K5.
I know this is heresy, but the reason I have a NEX is not because I cannot afford an M9.