I was wondering how accurate the focus peaking is in the NEX cameras, specifically the NEX 7. I am sure it will yield sharp focus but I guess what I am more curious about is its sensitivity. For example, if you had a razor thin DOF (say with the Noct) and you're shooting a portrait, would you be able to visually see the difference in highlighted areas (on the back of the screen with focus peaking) between focussing on the person's eyes/nose/eyelashes/iris?
I find it works very well for focusing on the iris.
There a few tricks to focus peaking. The first is dialing it in. Not only does the peaking level influence the amount of peaking, but so do your jpeg contrast and sharpness settings. I prefer Neutral creative style with -3 contrast, + 3 sharpness, and peaking level set to "low," but YMMV.
The second trick is what to look for. You basically have to look at a single point when focusing, rather than viewing how the peaking influences the whole scene. Imagine that you just have one AF focus point, like on a DSLR. When you narrow in on looking at just the iris, and ignore the peaking in the rest of the frame, you can watch the peaking swell up and down in the iris and shoot when it is at its highest.
With the above recommendations, I rarely need to use focus magnification. I think a lot of users get confused by peaking, because there can often be peaking all over the scene at various levels. Ignore all of that, and, just like using single point AF in a DSLR, focus on one little point in the frame and how peaking affects that little point, and you'll be fine.