I agree with your friend.

a demo certainly helps, but it's still far from reality.

Batman demo was good, but not astounding. However the full game is GOTY worthy.

so your saying it doesnt mean anything?

so go ahead and play the "hail to the chimp" demo and buy the game. expect it to be a first person shooter and be the best game ever...ur logic fails

You can't tell if games are good, but you can certainally tell which ones are baaaaaaaddddd.

exactly, that sonic in 3d was bad, and he says im a dumbass cause i said that. He never even played the demo though

i would say there is no definite answer. A bad demo could make a good game look bad, and a good demo make a bad game look good. I usually try demos, but usually I try to borrow or rent a game before I buy it unless its a sequel to a game I already enjoy.

I agree with your friend

Neither of you are really right. A good demo does not mean the game will be good, likewise with a bad demo-bad game.
Demos, by their very nature, will give you an impression of the game, but only playing the game itself will let you know how good/bad it actually is.
Your friend is certainly wrong though if he thinks demos are pointless, they certainly help.

Demo's never show anything about the full game, thats why i only play demos for games that i can't wait to play (i am doing it WRONG i know)

exactly because oherwise why would they even have a demo if it was pointless