This diagrammatic map pretty much ignores geography in the interests of making the other information present that much easier to decipher. It is given a very clean and easy-to-read look by eschewing the messiness of reality and only using lines that are horizontal, vertical, or at a forty-five degree angle.
However, since it does ignore geography to highlight the Tube's lines, it is fairly easy to be mislead into getting a distorted sense of the city itself while using only this map. For instance, looking at this map, it's not really possible to tell whether the stations are geographically very close to one another. One who doesn't realize that might spend half an hour in the tunnels when the trip above ground would have taken only five minutes, or vice versa. Over the years, some have felt that this was a failing, while others maintained that it didn't matter, as the map's sole purpose was to help one get around in the subway system.
Those who felt that geography oughtn't be ignored have tweaked and prodded at the map over the years. One such re-imagining of the Tube map can be found here. It's still easy to read, and it takes geography into account to give a much more realistic view of the system. Here's a zoomed in screenshot:
Check out their blog for news, links, and a chance to provide feedback.
If you're interested in the evolution of the maps of the London Tube, this site has a nice collection of maps throughout the years.
And here is a very interesting conversation from Edward Tufte's forum about London Tube maps, as well as maps for other transportation systems around the world, that spans nearly a decade. (Unfortunately, some of the older links don't work.)