The only way to get around London. The roads are cr*p and therefore buses are useless. Tip – get yourself an oyster card at the beginning of your trip, saves having to mess about buying tickets for every journey.
One of the definite upsides of hosting the Olympics is the improvements in transport. Just look at these snazzy new trains on the Metropolitan line. You can walk from one end of the train to the other. A lot less claustrophobic.