Bought for nostalgic reasons: I saw this film long before moving to London, then when living in London everytime I saw someone on a moped with a clipboard I thought 'The Knowledge!'. Last time I was in London, this summer, I saw the mopeds with the clipboards again and decided I wanted to revive my memory, so I bought the film. It lived up to my expectations, which can't be said for the cabby whom I once asked to drive me home after a late night at work, only to find out that he did not know the address (in South Ken.).
If you like London, you'll like this film.