For those who can't play it I'll give you my five minute experience.
Two maps, Lockdown and Shock... something. One is in Nova Prospekt, you know the jail. It is almost exactly taken from the campaign. A few of the gates are open or closed, and a ladder or two and some debree added for more maneuverability, but practically the same section. It is all indoor so it feels a bit crowded, though the big shower area is part of it and that's fairly open. There is an added health and energy dispenser. Your energy can go up to 200.
The 2nd level takes place in the city. I have not gotten that far in the game so I can't say if its from a specific place or not. You can go in a couple of the buildings, streets are blocked off by the regular combine walls. There are a couple cars in the streets.
Both maps have random physics objects all over: toilets, heaters, boxes, bricks, barrels, explosive barrels, tv's, tables, chairs, etc... You are given the gravity gun and you start with it equipped. You can kill people as easily with it in multiplayer as you can in single player. It is a bit harder to hit people because they move, and there is often other debris in the way. You can catch incoming debris with your gravity gun, you can use stuff as a shield. The five minutes I played I did not feel that the gravity gun is overpowered though. "Reload" is fairly long and it's easy to miss. But it is a nice one hit kill for areas where your enemy can't move around.
My one complaint, this may be because of lag, is that due to the visual debris (non physics, rocks and rubble) i had a bit of trouble walking around in some areas. I would get stuck easily. You of course must maneuver around boxes and chairs and things that are strewn all over, but it was frustrating to have to deal with rubble on the ground.
I generally despise DM, due to its lack of teamwork and strategy, but I may play this a bit, if only to think up ways the physics could be used in more teamwork oriented games.
(valve needs to add a see through riot shield that you can pick up with the gravity gun. It's impossible to see anything when you use a barrel...)