[quote name='lolwut?']Well, I really enjoyed the demo, but can anyone tell me if there is any replay value in here at all.
I know there is a co-op mode (can this be played offline?), but once you beat both modes, are you pretty much finished?[/QUOTE]
There's a lot of content just playing through the levels once. Most people would be between 5-10 hours depending on your skill (for single player). After you beat it, you can go back and look for the cake in all the levels. The game is designed for time trials (if you are into that). And then you get an achievement for beating hardcore mode (1 shot kills you, no checkpoints within individual levels).
You can do co-op offline for up to 4 players (I've read, I haven't tried it).