This thread seems to have split in two directions. As a long time former resident of San Francisco, I can't imagine anyone ever suggesting to any tourist to have a car in the city. That simply makes no sense assuming that the tourist is staying in a major tourist area, as Don clearly is.
Why are people now discussing the quality of the public transportation system as it applies to residents? I'm sure the original question was simply in reference to tourists, and San Francisco's public transport is perfectly clear, inexpensive, and easy to use for tourists.

So the bottom line is, don't keep your car Don!

I've decided not to have a rental car in SF solely because of the cost of the rental car plus cost of parking.

I'm taking public trans from SFO to the Hilton Towers. Where can I buy my 3-day Muni pass?

Mike,

According to MUNI's website, you can buy a 3-day MUNI pass at a ticket desk in SFO's Baggage Claim area. You can also get them at the Montgomery St. MUNI/BART station ticket office.

Here is a link to MUNI's 3-day pass vendors:

http://www.sfmuni.com/fares/farezip.php?prod=3-Day