OK, I thought riding around in a double-decker bus with a guide yapping on about London might be a cheesy thing to do, but I was wrong. I'll admit it! The Big Bus is big fun on a nice day. We rode along with London natives who were taking the day off to explore their own city. Can't argue with the locals!
You can buy your ticket online or at one of the designated bus stops (see their website). Once you buy your ticket, you can ride along all day and hop on and off where ever you like. The upper deck fills up quickly but offers the best view on a clear day. The tour gives you an excellent overview of the city and its history.
There are three different routes you can take. You really need a full day to take advantage of all three and the various sites you can take in. The bus runs by Westminster Abbey, Tower of London, Marble Arch, Picadilly Circus, Houses of Parliament, Buckingham Palace and others.
If you had only one day, or a limited time in London, I would say the Big Bus is a must if you want to take in the major sights of the city.