It's a good way for new Source developers to get entities, SWEPs, props, maps, and the rest of the bunch noticed in an easy way. Plus, it can be a great film making medium.
However, I never got into it. Didn't like multiplayer: Trouble in Terrorist Town and Zombie Survival is full of complete dicks, don't like RP and sandbox was generally faffing about, which I did anyway and got bored with in singleplayer.
Put less than 30 hours into it, and I haven't touched it in months.