The major drawback of this map is that it gives you no clue as to how to actually get to the nearest subway station, and it doesn't tell you which subway station is close to your destination. By focusing only on travel time it has abstracted away a bit too much of the information that people expect to get from a subway map. (For example, the NYC subway map shows the locations of subway stations with respect to major streets and landmarks.)

Also, in most transit systems, travel time can depend significantly on the time of day you're traveling. Express trains don't always run and the time between trains increases during off-peak hours.