I take super shuttle a lot, used to take south and east bay - but their prices went up and service went down so I switched.

There is a lot of variability due to the driver (but this is true of taxi as well), so it's a personal choice based on value/cost/etc.

I usually travel alone and the shuttles are the cheapest option for me - but from where I live if it was for 2 people a taxi would be about the same cost. SFO taxis charge a 50% surcharge once you exceed a certain distance from the airport if you get the taxi there (at least that's the way it used to be) which you can avoid by called for one to pick you up instead of using the taxi que. Maybe someone who takes taxis can confirm if this is still the case.