The significance of Harry Beck's map design of 1933 is widely recognized as one of the seminal works in Graphic and Information Design, and a major influence to all subsequent underground maps of the world. It's only when you see it next to its predecessor diagram from 1932, with its convoluted geo-based arrangement, that the brilliance of Beck's design really comes to life, in a diagram that is as fresh and original as if it had been created today.
Many people are probably not aware that Harry Beck's prowess did not remain within the UK. In fact, in 1951 he would submit a revised edition of a map he had worked on for the Paris Metro in the late 1930s. This map was eventually rejected by the Metro administration and would end up being largely forgotten. On March 2009, Mark Ovenden - author of Transit Maps of the World - made a great post on the Creative Review Blog, describing this important episode in modern cartographic history.
If one wants to see this map up-close, the best place to go is the London Transport Museum - a treasure trove to London Underground aficionados.