Focus peaking was one of my biggest concerns before I got the A7 because I planned to use only older MF lenses. So I'll try to add some of my thoughts, which hopefully end up making sense:
Overall - I tend to use focus peaking 99% of the time because I've been shooting people candidly. I've also been using mostly wide open apertures because I've been playing with the A7 indoors at night. Right now I get good hit rates, but I anticipate I will get excellent hit rates with practice. I started out with rangefinders last year (so the learning curve is fresh in mind) and so far I personally find focus peaking to be perhaps easier to learn. Either method though has its pros and cons and comes down to muscle memory and experience.
What I'm about to say might sound like "bend your knees" or "keep your eye on the ball" but hey - gotta start somewhere... The way I go about focusing is imagining the DOF plane as indicated by the peaking highlights. More than that - imagine the center of the DOF plane where critical focus is. It's not as simple as it sounds when photographing organic shapes strewn at different distances throughout the frame. A scene like that will have only a small and/or sporadic amount of peaking highlights. Combine that with simply not being critically aware of orientation with respect to a subject; I was surprised how often the swath of peaking indicated the DOF plane was at an entirely different angle than I imagined.
Anyways, with practice I like to think my DOF plane imagination has gotten much better because I am making much more purposeful focus adjustments than when I first got the A7. In other words, I seem to read the peaking better even when random things that aren't even connected to my subject are peaking.
Some other observations - just because something is peaking doesn't mean it's in focus. That may sound like a bad thing but it's not. The peaking starts off thin and flickering and gets more intense the more in-focus something is. This actually is generally helpful. E.g. when focusing on someone's face. You usually want the eye in focus. As it turns out most people's faces don't have many contrast lines for the peaking to show up on except for nostrils and, 9 out of 10 times, there's a tiny highlight reflected off the eyeball - perfect. If peaking were either full-on or full-off while at the same time the eye and maybe nostril have a pinpoint of peaking, there'd be no way to tell if the focus plane is just starting to touch the eye, or if it's centered on the eye (critical focus) because peaking would just be "on". But because peaking isn't like that, I focus until it's most intense and can generally tell when just the pinpoint of the eyeball highlight is critically focused by feeling the "bump" of the focus peaking. Yeah, it probably seems the peaking is too vague to do this, but it's not if I actively concentrate on making this happen. I've done comparisons using this method vs focus magnification and I definitely can determine critical focus quite quickly just using peaking. I'm not saying I'd never use focus mag again - just that practice goes a long way and most of the time I don't have time for focus magnification.
Last tip I have is I made the down button of the scroll wheel change peaking intensity. That way I just press repeated times (without changing buttons) to change the peaking intensity. Low light peaks less, so I use the high setting. Daylight peaks more, so I use the low or medium setting. Also stopping down has a similar effect.
Good lord I hope that was helpful. I was a nice mental exercise for myself at least.
