I've been using this effect like this for a year and it's quite nice when e.g. taking a video full screen or starting a slideshow. I actually became so accustomed to it, that I didn't notice it anymore, until I did that on a different computer without it which is why I now chose to upstream the change :D
I noticed going full screen in Firefox with CSDs on is a bit flickery but that's just CSDs for you right there, works fine with proper window decoration enabled. Or, actually, could be an issue in our extended frames handling when switching between window with shadow and full screen window withour or I don't know.
Also, for some reason the first time a window goes full screen does not animate