The HTML5 player is fantastic and I have been using it for as long as I can remember (since it became an option). My favourite feature is the playback speed controls, when trying to learn a song on guitar, I can follow along with the live performance or tutorial to learn how to play, absolutely invaluable.
Every day the number of reasons to keep Flash installed gets lower and lower. I still keep it installed on my machine for the occasional site that still uses it, but Flash's days are definitely numbered. When a site as big as Youtube stops using it, that is when you know there isn't long left. Fortunately Adobe realised HTML5 is the future a while ago and have been creating some great tools.