depends on the game. WIth sports, fighting, fps and a few others a demo can give a good grasp of how the game is. However with RPG's, strategy, heavy story based and such demos are not that great as they often leave you confused or show parts of the game with ether extremely weak and simple enemies or throw you into the hard ones without any idea of how to play
Basically the more complex the game is the less chance of the demo being a good judge of gameplay
Basically the more complex the game is the less chance of the demo being a good judge of gameplay