The Metro is the quickest way from one point to another, but I use it only when I'm in a rush. I try to avoid it all other times for a number of reasons. One is that I don't like almost running into a man peeing on the wall while I'm turning a corner to exit. Another is that it seems to be completely acceptable to ride the Metro and dig deep into your nose the entire time because EVERY time I'm on a train, I see this. Oddly enough, wearing gym shoes on the Metro will get you more dirty looks than the nose picking guy or girl. And, if those aren't enough reasons, having a crazy, drunken, toothless woman yelling at me in French while I'm waiting on the platform, will definitely do it. (Of course, she is the only one who can't figure out that I'm a tourist and cannot understand slurred French.)
If you've used the Metro and have never seen any of the above, you're very lucky. I've, unfortunately, run into these situations on more than one occasion, making the bus my first choice in public transportation. The bus also gives you a chance to actually see the city as you travel, and it's good for the elderly, less mobile, or people with luggage because they won't need to climb up and down stairs like they would in the Metro.
A two day pass for all public transportation within Paris (including the Metro and the RER) is 11.60 euros, which is quite a bit less than the 32 euro two-day pass that the open bus tour companies charge. (Just think, you could buy 5 individual pastries with the money saved!) The front of all buses have their line number and destination, and each stop is electronically announced inside the bus, making it very easy for tourists.
I've spent the last couple of days riding and seeing the sites. Here are a few bus routes that let you see many of the highlights of Paris.
Bus Line #27 -- (Gare St. Lazare to Port de Vitry) See L'Opera Garnier, travel down Avenue de L'Opera, see the Louvre, ride along the Seine, see Notre Dame, La Sorbonne and The Pantheon)
Bus Line #73 -- (Musee D'Orsay to La Defense) Ride along the Champs Elysees to La Defense.
Bus Line #87 -- (Champs de Mars to Port de Reiully) Travel from the Champ de Mars and the Eiffel Tower past Invalides, the Bon Marche, down Saint Germain des Pres and the Latin Quarter, crossing the Seine and seeing Bastille.
Bus Line #84 -- (Pantheon to Port de Champerret) Ride through the Saint Germain des Pres quartier, to Place de la Concorde and Madeleine)
Bus Line #30 -- (Gare de L'est to Trocadero) You can visit the Saint Martin Canal, then hop on this line to ride through Montmarte, the well-to-do 8th arrondissement, around the Arc de Triomphe and end at Trocadero to see the Eiffel Tower.
Bus Line #72 -- (Hotel de Ville to Parc St. Cloud) From Hotel de Ville, ride along rue de Rivoli and the Louvre, the Seine, see the Eiffel Tower, ride through the 8th & 16th arrondissements to the tranquil outskirts of Paris--from there you can take Tram 2 to La Defense and catch Line #73)
You have several options when purchasing tickets:
1) Navigo---for one week it's 16.80 euros and for one month it's 55.10 euros (The weekly ticket begins on Monday and ends Sunday at midnight; the monthly ticket begins the first of each month.) You also pay 5 euros to buy the Navigo card, and you need a passport-type picture of yourself to add to it. You can keep the card and reuse it each time your in Paris. This is worth it if you are spending 3 or more days in Paris.
2) Mobilis---valid for an entire day on the bus, Metro and RER; it's 5.80 euros per day.
2) Carnet--10 one-time-use tickets for 11.40 euros.
For Paris transportation information, you can go to www.ratp.fr.