BART is very safe, but damn slow.
Make sure the BART is running during your flight times. Early mornings on weekends are problems.

Allow 2-2.5 hrs between OAK & SFO with all the connections.

OAK AirBART $2 (comes every 10-20 min)
BART Coliseum to SFO $5.80 (50-55 min, comes every 20 min)
SFO AirBART Free

http://www.bart.gov/