I'm with haywiresf - I've taken it dozens of times in both directions (SFO-OAK, OAK-SFO) at all hours of day and evening. The actual train trip itself (SFO to OAK Coliseum station) takes 51 minutes and costs $5.80. Then the AirBart bus from Oakland Coliseum station to Oakland International is another $3.00 (as of March 1) and another 20 minutes maximum. If there is any highway traffic at all, there is no way you will do it faster or cheaper than BART. I've ridden BART twice a day for years - it is far and away the best transit system in the Bay Area.

Check your flight times carefully though - BART does not run after midnight. Starts at 4am on weekdays, 6am on Saturday, and 8am on Sunday.