San Francisco rents are bad. San Francisco purchase prices are balls-out disgusting: 30 to 50 years' worth of rent. Typical in the US is 15, and that's only because of the mortgage interest tax credit (which drives purchase prices about 30-50% higher than they otherwise should be).

Buying at current levels would only make economic sense if you expected rents to keep going up. Right now, buying is a bad move unless you expect the extreme rent situation to worsen (rather than mean reverting) in the long term (20-30 years). Given that it'd take just one genuine progressive mayor to crush the NIMBYs and allow new development, I think that that's a bad bet.