I haven't touched the single player but this is a very fun multiplayer game. For that alone it's easily worth $20-$30. Most of the multiplayer modes are great fun and inparticular the 2-4 player castle and tiki tower topple modes make for some intense matches. I can only imagine the single player isn't that good but I'm just assuming and I doubt it really matters. Buying a game like this for single player would be like buying Unreal Tournament for the campaign.
Demon's Souls Official Thread | Currently playing: Left 4 Dead 2, LittleBigPlanet 2, Magicka