I think the hop on hop off tours are a good thing to do at the beginning of a trip, especially if you have never been to an area before and your time might be limited. It gives you a good overview with highlights of many of the important sites. I have used Olde Town Trolley in other cities, and it does not disappoint. You can see what sites you may want to more thoroughly explore during your time there.
The San Diego tour provided a good starting place - and covered a lot of ground- even to Coronado. Be sure to see the grand Del (hotel) there and the great beach!
Other areas like Balboa Park (and the zoo), Old Town, Little Italy all had stops. I stayed in the Gaslamp district and there was a handy stop just a few blocks away. There are several other stops near other hotels as well. The tour drivers were all very nice and had good information.