This will hopefully be my third trip out to San Fran for VMworld and I have never used a rental car. The BART is a great way to get from the airport to the general vicinity of the conference. From there there are taxis and cable cars for anything that isn't within walking distance, but most things are. Fisherman's Warf is great and easily accessible by cable car. One tip I did learn from one of the locals is that the cable cars stop at most intersections to allow people to get on or off, this is a much better place to get on a car rather than waiting in the log lines at the ends of the runs.
As far as hotels are concerned, this by far is the most expensive I have seen the hotels in this area. $500 a night is a bit unreasonable and is far outside of my companies travel policy. I was able to find some hotels a little farther out for around $200 a night and they were still well within walking distance. Word of warning, I stayed at the Mosser my first year in San Fran, and while its a nice and affordable hotel, it has a single elevator, no A/C and unless you ask for something different you may end up sharing a bathroom in the hallway.
Hope that helps.
