The demo is one of the best ones I've ever gotten the chance to play. It's only two chapters of the Parish campaign, with the first one (the Docks) being completed pretty quickly.
It's insane just how many special infected there are while playing through the levels. On normal there was always at least two witches (one wandering and one staying put) and two tanks.
The new special infected are fun. I rarely saw any hunters or boomers until my last few run throughs. The spitter is evil, always spreading her toxic spit either right where you need to go or so that it traps. The jockey can get annoying when they come at you constantly, but they aren't too hard to deal with (though their size makes it hard to pick them off).
The new weapons are great. All of them handle well and are fun to use. Each is different, though a few that are in the same class feel like there is very little change. The assualt rifles are certainly the most fun, though they still eat up ammo like before. The melee weapons are fun with each having its own pros and cons. They do timing when it comes to hitting them though.
The bile bomb and defibrillator are useful. The bile bomb cause whatever it was hit by to be swarmed by zombies (useful for tanks). The defibrillator was used on me after a hunter pinned me down. After you die or a friend dies you have a certain amount of time (30 seconds maybe?) to run over to them and use it on them. I have to say I was yelling at the screen for the guy who had it to use it on me (and he didn't disappoint.)
All in all it was a great experience and I know I'm going to be playing it with everyone over and over until L4D2 comes out next month.