Demo is great fun but very short. You get to try part of the campaign with a big battle at end of level. Melee is awesome, you have to stun (F) and then execute (E) enemy to regain health. Which means you keep getting stuck in, you do have to time your executions though as you still take damage from other enemies.
The other part of the demo you get a jetpack and can try out the aerial attacks and flying around, multiplayer is gonna be awesome I think.