I would like to hear some opinions/experiences about car rentals in San Francisco and LA. Tell about service, expense, etc. I want to pick up a car at the airport and return it downtown.
In other words which company is the cheapest without compromising service.

You Do Not want to rent a car for SF. Take a van or taxi from the airport and MUNI or taxis in the city. Only if you want to see the wine country, rent a car while up in the city for one day. parking at some hotels is $35 day and is impossible on the streets. For LA you do need a car. I would rent at the airport and return at the airport and check Priceline for prices---all companies are basically the same.

Ditto kamq's message!

Ditto the above advice. If you're not comfortable with Priceline, check www.bnm.com . Lets you easily compare rental prices. Rates vary on a day-to-day basis. Generally speaking, the big two (Hertz and Avis) have a slight edge on service.

I agree with those who are saying you need to shop around. I used Enterprise when I was in LA a few months ago; they were the cheapest on the day I booked. I must admit they were a bit dazed - they "couldn't find" the car I had booked, so they gave me a much larger one,which they considered an upgrade, although I considered it a bit of an inconvenience. Then, when I returned the car, they couldn't find the guy who worked in the office (a desk in the lobby of one of the airport hotels). Eventually he turned up. He spent some time trying to arrange for the shuttle to the airport, despite the fact that I told him at least twice that I wasn't going to the airport! Having said that, everybody was pleasant enough and the car worked fine. I wouldn't hesitate to rent from them again if their rates were good.