Before I use that marvelous ability, I normally check demos of games I don't know.
"Hey, seems a good game, but before I waste my time and Internet channel on such a large game (every game now go from 1GB to 8GB), let me test the demo, if it's fun I might aid violating their patent rights.
Well, games nowadays, specially the multiplayer ones, have another way so that you don't access the good stuff. Like Battlefield franchise, if you don't have an original, you can still play either sp or mp, but not the ranked ones...not the good stuff. There's so much difference between legal Battlefield and the other one that buying battlefield is totally worth it.
If I had played SimCity Societies Demo, or Bioshock, I might haven't bought the game.