Unfortunately, Flash still plays Youtube videos more efficiently than HTML5. On weak hardware this can make a difference between smooth playback or choppy video or watching videos in much lower resolution that you normally would with Flash.
Maybe that's true on Windows? On Linux the HTML5 version of youtube is way more reactive, feels(!) to download faster, and the GUI itself is broken less often (flash player often had some broken lines, or some elements that seemingly haven't been in the right place)
My experience is for Mac OS X only. I don't use Windows much and don't watch Youtube on Linux.
In what browser? I find Safari with HTML5 video much better than Flash.
I use Firefox latest version and Safari 7.1.x (most recent version available on Mavericks).