I have only ridden the Supershuttle once before – usually on East Coast we have a car waiting.
Based on these opinions: http://www.propinions.com/opinions.php?s=9718545&page=4
Supershuttle offers a good service at a good price. My trip from BWI Baltimore to a house about 20 miles north is only 24+tip (interestingly their online form requires the tip field to be filled out. I don’t like that since I’d prefer to tip based on service, but …)
Rather than book online I think I’ll wait until I’m at SFO and confident of my arrival time. I’d hate to prepay and then have delays, though I’m assuming they’d refund if you can’t use the service?