I played the demo as well, several times in fact, and I have to say that its a lot of fun. However, I don't think I can justify $60 for a game without multiplayer, or at least a sandbox-like environment. Assassin's Creed had the open environment I crave, but after I beat it without multiplayer it was pointless.
That being said, I'll probably rent The Force Unleashed and blow through the campaign, but unless there's multiplayer or co-op or something, it's not worth my money.