This is a really nice way to see the city. We booked the 3-hour tour, saw the golden gate bridge, the painted ladies, the croockedest street and many other features of San Francisco. All with great support of the GPS guided tour. You can do it all on your own speed, there are several choices on extra detours or cut backs, this way you can make it your own city tour, with the things you're the most interested in.
On our tour, several people asked about the gocar, where to book it and about the costs. They said it looked like great fun, I don't know if they meant the gocar or the big smile on our faces :-)