Moving to Washington D.C. was the first time I had ever had to master some form of public transit to make my way around, and I think everyone can appreciate the sense of championship accomplishment that comes from memorizing the metro map and making your daily commutes with ease. In Paris, I not only get to explore much more of the city to make it to classes every day, but the public transit system is infinitely more confusing and colorful. To compare:
My Washingtonian habit of describing all lines by their color becomes much trickier in Paris. Friends are much less receptive to following the "periwinkle to the mauve to meet me" and I have no desire to explain to someone that I live at Pereire on the "pea-colored line." In addition to 16 colorfully-numbered metro lines, you have 5 train lines, 3 trams (found only in the south), and dozens of buses. Like Washington, these shut down around midnight or 2am, at which point a delightfully confusing "noctillien" system of night buses kicks in, which may or may not take you where you need to go based on how closely you can read a bus map. With a combination of timing, skill, and dumb luck I have caught countless Friday metros at the last minute. And I have conversely been forced to take almost just as many late-night bus rides with an eclectic mix of the Parisian populace. Public transit is the great equalizer in many cities, and nothing will bring a group of Parisians together more than a metro car stuck underground due to track maintenance or delays.
While the payment system for Paris is much easier - you refill your "navigo" with a flat rate each month that gives you unlimited rides on every form of transit - the standards of cleanliness are not the same for each station. The Paris metro has been operational since 1900 and its Art Nouveau signage really is an iconic part of the Paris landscape. Because of this you will also likely encounter many stations that look as though they have not been maintained since 1900 and could do with some insulation, ventilation, and casual stalactite-removal. Metro stations are a popular spot for bakeries, news stands, street performers and singers, and in some stations entire underground malls. No matter the time of day you can easily find hundreds of Parisians at every station, though a certain social code of quiet talking, no eye-contact, and little physical contact is always observed (by the French at least, you can always tell a tourist in the metro).
Every Parisian has their preferred mode of transit. Buses are sometimes more direct and allow you beautiful views of the city. I will always prefer the metro for the simplicity and the adolescent thrill of metro-surfing, as well as the ability to discreetly people-watch a wide spectrum of the Parisian populace. My friend Diego however swears by the RER system, which extends out from Paris into the neighboring suburbs and countryside, and thus gets to enjoy a much wider slice of French commuter culture. With any system you run the risk of delays, missed connections, unfortunate mysterious smells, and being asked directions in any number of languages. There's a certain pride that comes from knowing my regular metro routes, but I'm nowhere near knowing all 245 of Paris' stations. The beauty of it all is that one can find a metro almost every block - it really is one of the densest public transit systems in the world - and so when the weather is nice it's almost not worth the trouble. Paris was a city made for walking, and though this spring is one of the latest in French memory, I look forward to the day when I can stroll along the Seine, not roll along under it.