It's a shame game demos are not released prior to the full game release anymore (which indeed makes much more sense to consumers, no matter what developers say). In any case, better late than never, Valve has now released a demo of the game weighting in at 760 MB approximately.
The demo takes players through some of the early levels of the game and later transports them to the area of the game known as Ravenholm.
You can download here, or choose from a complete list of mirrors.