We use the Paris metro often, whenever we need to go somewhere that’s too far to walk. The system works well and isn’t very expensive. Now that we’ve been here for a couple of months, we have gotten the hang of how to navigate (thanks in part to Google Maps), we really enjoy the trip.
From Wikipedia:
The Paris Métro is a rapid transit system in the Paris metropolitan area, France. A symbol of the city, it is known for its density within the city limits, uniform architecture and unique entrances influenced by Art Nouveau. It is mostly underground and 214 kilometres (133 mi) long. It has 302 stations, of which 62 have transfers between lines. There are 16 lines, numbered 1 to 14 with two lines, 3bis and 7bis, which are named because they started out as branches of lines 3 and 7; later they officially became separate lines, but the Metro is still numbered as if these lines were absent. Lines are identified on maps by number and colour, and direction of travel is indicated by the terminus.
It is the second-busiest metro system in Europe, after the Moscow Metro, as well as the tenth-busiest in the world. It carried 1.520 billion passengers in 2015, 4.16 million passengers a day, which amounts to 20% of the overall traffic in Paris. It is one of the densest metro systems in the world, with 245 stations within the 86.9 km2 (34 sq mi) of the city of Paris. Châtelet–Les Halles, with five Métro lines, three RER commuter rail and platforms up to 800 m apart, is one of the world’s largest metro stations. However, the system has generally poor disabled accessibility, because most stations were built well before this became a consideration.
The first line opened without ceremony on 19 July 1900, during the World’s Fair (Exposition Universelle). The system expanded quickly until the First World War and the core was complete by the 1920s. Extensions into suburbs and Line 11 were built in the 1930s. The network reached saturation after World War II with new trains to allow higher traffic, but further improvements have been limited by the design of the network and in particular the short distances between stations. Besides the Métro, central Paris and its urban area are served by the RER, developed beginning in the 1960s, several tramway lines, Transilien suburban trains and two VAL lines, serving Charles De Gaulle and Orly airports. In the late 1990s, the automated line 14 was built to relieve RER line A.