To us, demos seem like a great way to get the word out about games, but a recent study suggests that they might be better tools for consumers than solid marketing strategy. The Electronic Entertainment Design and Research Group (EEDAR) reports that games that are marketed on Xbox Live and PSN with a trailer only sell considerably better than games that ship with a demo.
On the PS3, games with no demo or trailer actually sold better than those with a demo, while the inverse was true on the 360. Now, this doesn't mean EEDAR wants you to lose your precious demos forever, but they recommend starting work on a demo after the game's on shelves so no polishing resources are lost. Though we can definitely see the logic in this (we've been kept from buying a few bad games by demos), we kind of wish EEDAR would keep our secret to themselves.