Personally I enjoy the demo, too bad it'll prob be deactivated when the game comes out
My only problem is with the difficulty spike, that or me and the rest of the peeps I play with suck. Most of the time we can breeze through the first level on Expert, then get our asses handed to us 9/10 times in the Park area of the second level. However, these repeated deaths allow me to watch more pretty corpse explosions from a well timed Pipe Bomb ^.^
Special Infected are definately smarter, I've seen a charger, smoker AND Jockey come and kick our asses, final insult to injury was when all those three were cleared, and the tank came to drop the last of us.
The witch however seems to suck, she just runs off the map half the time!