Been watching since it split from JustinTV, I started with Halo since that's my favourite game but then I started watching any competitive game like Starcraft, CS, League of Legends, racing games etc. I just enjoy watching games being played at a high level. I used to livestream my F1 leagues and usually had a good group of viewers who just enjoyed watching competitive racing.
Anyone can stream any game, so there's always a huge choice of things to watch. Whenever a new game comes out there's usually people streaming it on release day so you can have a good look to see what it's like before buying it. There are also some good discussions and hangouts on gaming news, things like the Co-optional Podcast every Tuesday.
Amazing to see how much the site has grown over the past few years. A LoL stream pulled just under 800,000 viewers during the world championships a few weeks ago, and that was just the English stream.