Probably the best example personally of a demo making me want to buy a game was Half-Life: Uplink. I got it in a copy of PC Gamer years ago, and I had seen Half-Life in the store, but I had never really given it a second thought. Thought it looked kind of boring. Then I played the demo, and instantly fell in love with it. It did so many things I had never seen done before, scripted sequences, NPC's that you could talk to, and would follow you. Fixed machine guns you could actually use. Realistic weapons, and the grunt AI was amazing. A great demo can sell a game extremely well, a crap demo can put people off completely. Just make sure if you make a great demo the game itself matches up. Other examples include the Doom shareware, and Quake, although I knew I was going to get it, just didn't have the money for the full version at the time. It's kind of annoying that companies really don't do demos any more, I've find I buy fewer games now. Especially PC, since it's usually a crap shoot if your computer is even going to be able to play the game.
