As a huge endorsement for the demo concept, the Civ Rev demo sold me on the game. I wouldn't have bought it without having played a demo, but the game scratched an itch I haven't scratched in years. Offering a huge departure from the glut of action games, and offering a nice length of game that's pretty jump in- and out-able, when everyone else is buying MGS4, I got Civ Rev.
IMO demo's are one of the best ways to sell your product. I'm surprised how some are stupidly restricted or limited. give the player enough to get immersed, and then pull the plug and leave them wanting more!