BART goes directly into the SFO airport. If you are taking a flight from the international terminal, it is a direct walk to the check-in counters. Otherwise you

BART goes directly into the SFO airport. If you are taking a flight from the international terminal, it is a direct walk to the check-in counters. Otherwise you connect to the airtrain which is an escalator ride up from the BART platform. I've found it extremely convenient getting to/from flights from San Francisco, although I'm lucky to be within walking distance to a BART station from my apartment.