Originally Posted by sugarbutch
The small LCDs aren't quite as good as looking through the glass yourself. They'll eventually be good enough, though.
Right now, I find the main problem to be lag and noise in low-light situations for electronic viewfinders and glare for the screens that are on the back of the camera. EVFs, like on the Olympus OM-D E-M5, are pretty good when you have a lot of light, but they tend to get blotchy, noisy, and sometimes laggy (low frames/second) in low light. The technology is such that there is an annoying lag from when you press the shutter button to when the EVF goes live again, presumably because the camera is busy moving bits off the sensor for the shot, so it can't display them on the EVF. It can be annoying when shooting action, like sports. Ideally, with an EVF, there should never be finder blackout, and it's probably just a matter of time before computational power catches up to make this possible.
Speaking of action, the contrast-detect autofocus these mirrorless cameras have can be pretty slow, though they tend to be more accurate than the phase-detect systems used in the mirror SLRs. That said, the Olympus AF is fast enough almost all the time.
EVFs are also shut down when not in use to save battery life, and the lag to turn on the EVF can sometimes be too long. You can work around this with various camera settings and a change of some habits.
I don't think I could ever live with a camera whose only view is via a screen on the back. The glare in bright sunlight makes most of those things useless, and the ergonomics are pretty terrible, too. They are useful for certain things like getting critical focus with live view, but they shouldn't be the only option for a serious camera.