I'm trying out this distraction free writing mode on WordPress v4.1. It looks and works pretty sweet. It doesn't take over the whole screen, just the window that you are working in. Which is kind of nice because then you can still see other things going on if you need to and a quick mouse movement outside the text box brings up the standard WordPress menu.
Check out a quick video of how it works.
It's almost something they should just enable by default. I could see clients/users I interact with enjoying it but not likely to find it or even use it because it sounds "fancy" to enable a distraction free writing zone when you're just updating the company blog about a new widget that's on sale.
But it works for that level of writing just as well as it does for long form, thought-leader thought-pieces like what you'd commonly find on Medium.com.
I'd suggest you click the little expander "X" thing above your WordPress content block and try it out.