I think overall this is a good company with reliable service. Seeing San Francisco by open-top bus is a great way to get around, but don't count on getting any fantastic historical information or new insights into the area. The busses run every 20-30 minutes, so you can easily hop off for a quick walk around the neighborhood, or an extended stay in the area (lunch, museum, boat ride, etc).
The "tour guides" (separate from the driver) hang out on the top deck with a microphone, and they'll talk about things as you go past them. Some guides are better than others - you get to experience a variety if you do hop on and off a few times throughout the day - but for the most part it's, "Over there is the..." and, "If you look to your right, you'll see..."
Yes, obviously, that IS what I see. A chinese restaurant? A museum?! You don't say!
As a local Californian who spends many, many weekends in San Francisco, we figured it was about time we took one of these double-decker busses. There are 3 lines:
1) The "Downtown" tour - this is great if you want to see the main tourist areas of SF, *except* the Golden Gate Bridge -- shopping (Union Square), the Ferry Building (good food and boutiques), North Beach (aka, "Little Italy"), Chinatown, etc.
2) The Golden Gate Park tour - this will take you out to GGP (bigger than NYC's Central Park, I'll have you know!) and all of the wonders that it holds - Japanese Tea Garden, Conservatory of Flowers, the de Young Museum, the California Academy of Sciences (GO!), and more. Along the route are great stops such as Noe Valley and Haight-Ashbury.
3) The Golden Gate Bridge - Sausalito tour. Any trip to SF must - absolutely *must* include a trip to, and possibly over, the GGB. Pedestrian access on the bridge is permitted, so there's nothing that says you have to take a bus if you're not renting a car, but from the majority of the rest of SF's "touristy" spots, the GGB is a bit of a haul. Taking the bus over and into the cute little town of Sausalito ain't a bad day, no sir.
Unfortunately, there's not any one tour that covers ALL of the highlights of SF for an out of town traveler (coincidence? I think not.), but fear not - City Sightseeing has you covered with a "4 in 1" tour that gets you access to all 3 bus tours, plus a ticket out to Alcatraz for one price. If you have only 1 weekend in SF for the rest of your life, this is without a doubt the way to go.
Otherwise, I recommend you chose a tour that suits your style of travel -- Are you a shopper? A museum hopper? A midnight toker? Wait, that's a song...nevermind. A photographer? No matter which, you really can't go wrong in SF - it's a world-class city. There's just so much to see within its 49-square-miles, it's impossible to do it all in 1 day (or on 1 bus tour). Depending on which tour you chose, and which tour guide you get, the Hop On / Hop Off experience might be your dream vacation or anti-climactic, but no matter which, SF won't disappoint.
Pack lightly, bring the stroller if you have a little one, dress in layers (yes, even if it's summer - we just had high temps in the 60's in July recently), and charge those camera batteries. Otherwise, hop on, hold on, and go for it.