Its a fun game, I have had a few crashes, but overall it great fun to shoot those critters.
The main point of this is not the game itself though, it is a gift to the mod community.
Source SDK users have been stuck with the Orange Box version of the engine, with the release of this free game, indie developers and modders may have access to a post L4D2 version of the engine, without requiring them to own L4D2. (and more Steam users doesn't hurt either
This seems to be a growing trend for game companies, respecting independent developers, and realizing that users in many cases are as good as or better then the original developers.