The PC Demo has 3 chapters of Halo (all set in one of the 10 "Levels" i.e. maps, namely "The Silent Cartographer") and yet, I still can't decide whether I like it. Multiplayer Halo is pleasant, but not addictive like Counter-Strike. I guess it's not clear how to be a power-user in the game, something I always enjoyed about Half-Life-based games.
I ran into someone on a Halo PC demo server (all Blood Gulch, all the time) and he claimed he had bought the full version, but he couldn't find many servers for the full game-- just the demo. Curious!