Found the hop-on hop-off to be very reliable and informative. The buses came regularly and within the timeframe given and visit all the main attractions in San Francisco. The voucher needs to be validated at the main office, just back from Fishermans Wharf, before first use but that was easy to find. The office is very close to a stop, so you can use it straight away. The buses are open top, remember to take a hat and sunscreen..as some people were getting quite sunburnt..however we were prepared for this and did not find it a problem. Having visited here before, this was my first time using this and I highly recommend viewing the attractions by this method. It was a very exhilerating way to visit this wonderful city!!