I lived in San Francisco without a car for about 4 years. San Francisco is very small. It's possible to walk from the ballpark to Ocean Beach in about 2.5 hours. I often walked from my home in Cole Valley to my office in SoMa, even though I could just take the Muni.

The public transportation is pretty good for a city in the USA but pretty bad compared to Europe and really bad compared to Japan. People who like the public transportation in SF usually live near one of the Muni train lines. People who hate the public transportation are usually in a neighborhood where they are stuck taking the bus (which is most of the neighborhoods).

That said, having a car in SF is nice, too. The downside is parking. The upside is being able to just drive over to Safeway for groceries, or drive up to Marin, or throw a surfboard on the roof and go down to Pacifica or Santa Cruz.

You can get most of the way there with Zipcar, but I found that if you regularly want to get out of the city on weekends, Zipcar was not as cost effective as owning a beater automobile.

I just saw your edit. The public transit is not as good (comprehensive) as NYC but is a little better than Seattle.