I agree with fireme that I would consider most areas safe.
Statistics-wise, San Jose is generally considered the safest large city (> 500K people) in the US. San Francisco has roughly twice the crime rate of San Jose, which I believe puts it below average safety-wise. Oakland is really bad, but I have never even visited there outside of the airport and I have lived here for over 10 years. A good friend of mine was recently robbed in SF, and she suffered a concussion. But that is just anecdotal, of course, what matters more is the stats.
San Francisco recently passed an ordinance that will require all private citizens (including former law enforcement, etc.) except for law enforcement to turn in their handguns to the authorities by April 1, I believe. I find this unbelievable, but then, that is why I don't live there. It was being challenged in court. I went out to eat in SF on Friday evening and I was reminded just how expensive things are there -- we ate at an average restaurant that charged 50% more than a superior restaurant nearer to me, but these were friends from out of town. Living wage laws, high rents, and anti-biz regs raise the cost of everything in SF.
Kramer