Yes and no. Marin county was hit very hard in the last recession. Hard to imagine it now, but from 09-12 values were severely depressed. They have come roaring back in the last 2-3 years. The city itself plateaued, but never really dipped. Of course, anything could happen, but short of an international calamity or giant earthquake, it's hard to see the local market (city, silicon valley, probably Marin at this point) taking much of a dive. It can sometimes be hard to fathom the insane amount of wealth here, juxtaposed against a tight housing pipeline. SF is only so big; the whole region is limited by environmental protection, height restrictions, and NIMBY-ism. Yes, building is occurring, but not at a rate that is sufficient to satisfy demand. Add to that that every time a Twitter IPOs, 1000 millionaires flood into the market. Google, Facebook, Apple, and dozens of other companies pump out thousands of new millionaires per year. Add to THAT the intense interest from Asia; millions and millions of people with money who are smart enough to try to get it out of China.
Sure, anything could happen, but as one of my bosses likes to say, "100 years from now, people are still going to want to live in northern California."