FWIW, I live in Mountain View, have a car, and still frequently use Caltrain to get up to friends or events in the city. I'm within walking distance of the station, and during rush hour traffic Caltrain will often get me there faster than 101 will.

It's somewhat ironic (and telling about SF) that as a suburban commuter I've made better use of public transit than you have in the city, but it really is pretty convenient if you're going to any of the downtowns on the Peninsula, anything BART-accessible, or Candlestick Park.