I downloaded Mass Effect 3 demo and played through it today.
You can choose a couple of different options for gameplay. I chose Story mode mainly because I am not that great at FPS style gameplay nor do I really like it that much.
You get to try a couple of story segments.
1) An escape mission - You must get Shepard to the Normandy while fending off husks and a new creature during the Reaper invasion of Earth.
2) An escort mission - You are trying to get a female Krogan who is being held captive in a Batarian base onto the Normandy.
It is about a 30 minute play through. Very well done, this visuals are great.