Well the AirBart bus sucks when its busy. Also (this may have changed very recently) you have to buy your $2 ticket in advance for the bus - you can't just give the driver $2. And then you wander looking "where do I buy the $2 bus ticket?" Well, you just buy a BART ticket with a $2 value, collected on the bus. I missed a bus because of this. Allow 1/2 hour from station arrival to airport arrival, although it could be much quicker.