What Rob doesn't mention is that the public transport he recommends only comes once per hour and doesn't leave you anywhere near the tourist attractions in San Francisco. You still have a long way to walk. So his initial statement of 20mins to San Francisco is misleading - oh it's San Fran alright, but it's nowhere near anything interesting.
To get to downtown/tourist parts of San Francisco, you need to catch bus 51a to a dangerous suburb called Oakland (which is full of mentally ill people and drug addicts hanging around the bus stop) and from there catch the BART (Bay Area Rapid Transit). You need to do this in order to get into town and it takes a minimum of 1 hour per day (depending on how long you have to wait for bus 51a).
The BART and bus 51a are also your only option if you plan to get to his house from the airport using public transport.