This is an old post and the OP likely does not need this information anymore. However, others Googling for similar results might... so I thought I'd share this.
If you are interested in going downtown directly, are arriving in SFO during morning weekday rush hours, and would like to save a few dollars, the Samtrans KX express bus is also an option for $4. (It is not nearly as frequent as BART which runs for most hours of the day at 15 minute intervals, however. So you should check the schedules and decide whether it is worth the wait.)
From the timetables, you can see that there are no stops between the airport and downtown SF. Depending on traffic, it can be much faster than taking BART. Moreover, it's less than half the cost.
There is definitely not as much space as a BART train for luggage, but they did start allowing luggage again as of 2013: http://archives.sfexaminer.com/sanfrancisco/samtrans-bus-to-sfo-to-allows-luggage-again/Content?oid=2337301
Some more information regarding the route from local riders may be available on this Yelp review page of the bus: https://www.yelp.com/biz/samtrans-kx-san-carlos
tl;dr: The SamTrans KX express bus provides nonstop service to downtown SF for $4 if you arrive in SFO at the right time and don't have too much luggage. It can be faster than BART if traffic is not too bad.