If it wasn't for demos, I'd never have gotten into the Unreal series, which has been by far the highlight of my gaming life (and it was a multiplayer demo, too). I can't recommend them highly enough.
Same goes for Civilization, Half-Life, Doom 3, Age of Empires, Max Payne, Far Cry, Delta Force and a bunch more I can't remember at the moment. I've literally pumped hundreds of pounds into the games industry because of demos.