I know the subject of demo impressions has been beaten to death, but there's one thing I'd like to add.
I played an online co-op match with a friend of mine who is a long time resident evil fan, and can definitely hold his own. It made for a really enjoyable co-op experience: sharing ammo and supplies, formulating some pretty complicated strategies, along with just beating the shit out of zombies. I found that playing with another friend made the game much more fun, and both of us really enjoyed the co-op dynamic. We can't wait for the full version.
Demon's Souls Official Thread | Currently playing: Left 4 Dead 2, LittleBigPlanet 2, Magicka