After seeing WebRender Newsletter #9 today, I gave it a try in my rather new Profile (no extensions) and even scrolled through this topic. The first improvement I noticed is that I didn’t crash Nightly this time.
I am observing some rough edges:
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I get a blue screen for almost three seconds covering all of the area Firefox Nightly would occupy, before seeing the tab, navigation bars, etc.
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The Windows control in the upper right (minimize, full screen, close) are not visible, but I do get the mouse hint when I hover the mouse pointer over where they should be, and close (X) does indeed close Firefox. I tried several themes, but those controls still remain invisible.
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On the tab bar, the New Tab icon (the plus sign) is not visible with the built-in themes, but it is visible when I add a user theme.
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Typing on the search bar has an unpleasant delay after hitting the second character before the third character appears. Once the third letter appears I also have search results. … Oh, this happened just once per Firefox start-up; I wonder if it was waiting on render process creation or something else, but it was annoying.
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Library → Bookmarks → Show all bookmarks showed a solid black area for two seconds before displaying the bookmarks I have in that profile.
It just feels like it has a lot of jank, and hitting an apparently blank part of the top part of the Firefox window closing the browser (because the “X” is invisible) isn’t to my liking. 
For my overly-long Netflix disc queue, the one that took almost a minute in Firefox Release 53, took 31 seconds in my test Nightly 58 with WebRender enabled, but only 28 seconds in my regular Nightly 58 profile with WebRender defaulted to disabled.
I’m turning it off again in my “New” Nightly profile. (I’m glad I didn’t test in my regular Nightly profile.)