Just beat Dead Center on normal. Spent an hour and 39 minutes on it. The first chapter is pretty cool, especially going through the smoke and fire filled lower level. The crescendo at the gun shop was pretty awesome, mostly because you have access to all of the firearms in the game pretty much.
The finale... oh man. I don't know if it was just the people I was playing with or not, but it was difficult. First time we did it, it was basically a mad dash. We split up into two man cells and tried it out that way. We got owned. Then we tried to gather all of the gas tanks into one spot and someone accidentally set them all on fire. The third time, we went up to the third floor and got those tanks and threw them down. Then we'd fill up the car. Rinse and repeat for the second and first floor. We managed to complete right before the third tank came. You have to keep moving. Like BuddhaRockster, you can't corner camp. Even in the crescendo for the gunshop, you can't corner camp because you have to get the cola back to the owner.
It also seems that Valve has decided to connect each campaign, which I really like. It was pretty cool when they all introduce themselves in the first level.