I've taken a taxi from SFO to Millbrae only twice, when my flights were delayed and arrived after BART shuts down. I paid $11-$12 (including a hefty tip out of guilt for the small fare). I haven't noticed how easy/hard it is to get a taxi from Millbrae, however.

I have taken the Samtrans bus, but only from the Peninsula side. The advantage was that it was cheap and stopped directly at each terminal with no transfers. That saves the time, hassle, and expense of the BART connection. (Back when I used to park at BART, it could take me 30 minutes just to get from the airport to the BART garage.)