The only problem is that it has problems with some mkv files and oftentimes pausing doesn't continue without a little rewinding and fast forward first.
There is a fork of mplayer (the backend) and smplayer called mplayer2 and smplayer2 which I'm testing right now. This is what you need to put in the terminal to get it:
sudo add-apt-repository ppa:rvm/smplayer
sudo apt-get update
sudo apt-get install smplayer smtube smplayer-themes smplayer-skins