Before I explain why I am so in love with the electronic viewfinder on my Sony A7RII, I should probably explain what both viewfinder types are for those of you that don't know. All cameras have either an optical viewfinder (OVF) or electronic (EVF). Optical viewfinders are found on the majority of cameras on the market today. These viewfinders use mirrors to let you see exactly what the lens sees regardless of your camera's settings. Electronic viewfinders are the equivalent of looking at a mini screen that shows you exactly what your sensor is going to capture adjusted to your settings.
Right off the bat you can see why electronic viewfinders are great; They show you your depth of field and the exact exposure you're going to capture before hitting the trigger. With optical viewfinders, you will only find out what your final shot looks like after you take it.
Electronic viewfinders also offer something I appreciate on a daily basis; I no longer have to stare into the sun. I take a lot of sunrise/set pictures. With an electronic viewfinder I am looking at a screen image of the sun. In other words, I can stare straight at the sun and it won't hurt my eyes since, at it's brightest point, it's a white pixel. Since optical viewfinders use mirrors to look directly through a lens, you are literally staring at the sun when taking a picture.
Lastly, electronic viewfinders allow a camera to be mirrorless which brings the overall size and weight down. See the example below to see the difference between the Canon 5DSR (OVF) and the Sony A7RII (EVF)
Canon 5DS R vs Sony A7R II - Actual Size
If electronic viewfinders are so great, why doesn't every photographer use them? For one, electronic viewfinders have a specific resolution and quality which, on some lower priced cameras, can be low. Nobody wants to see a 20+ MegaPixel photo on a low resolution/quality screen. Secondly, some lower priced camera's electronic viewfinders have a slight lag to them. That's not a problem for landscape photography, but will be for any sports/action photography. Lastly, electronic viewfinders need to be powered and will effect your battery life.
While the cons sound significant, I couldn't be happier with the Sony A7RII electronic viewfinder. It's high resolution, has very little lag, and my battery life has never been a problem unless I forget to charge them up between daily sessions. If you can, go out and rent a Sony A7RII. I think by the end of the day, you will be blown away by what it can do. Let me know your thoughts below.