Taxi
When you get to the airport, get a taxi at one of the taxi stands. Don’t be afraid to ask other travelers if they would like to share a taxi into town. Fares can range from 38 euro (no traffic at all) to 75 euro or more if you are waiting in a lot of traffic. During heavy traffic, it might take you 2 hours to get to/from CDG and the meter is running the entire time. There is an additional charge of a euro per bag for two or more bags. A taxi is a good option if you have a lot of bags. Only take a taxi from a taxi stand that is clearly marked as a taxi.
Bus
There are two bus options:
Roissybus (http://www.ratp.info/informer/anglais/aeroport_roissybus.php):
Travels between CDG and Paris Opera Garnier (rue Scribe and rue Auber). It takes 45-60 minutes. It runs every 15 minutes between 6 a.m. - 7 p.m. and every 20 minutes between 7 p.m. - 11 p.m. Cost: 8.60 euro. You’ll need to take a cab/metro/bus from Opera Garnier to your final destination.
AirFrance Bus (http://transfer.airport-paris.com/air-france-coach-service.htm):
There are 2 routes to/from CDG:
Route #2: CDG->Porte Maillot->Arc de Triomphe. Cost: 13 euro 1 way, 20 euro return (R/T).
Route #4: CDG->Gare de Lyon->Montparnasse. Cost: 14 euro 1 way, 22 euro return (R/T).
There are signs as to where the bus will pick you up. Pick up locations at CDG are close to gate B1 and C2. Pay the driver when you get on the bus. Travelers with heavy luggage, disabled persons, or families with small children in tow may find this option to be uncomfortable.
Private Shuttle
There are a variety of private shuttle operators at CDG. They are usually chauffeur –driven vans or min-vans that will take you directly to your hotel, sometimes with stops in-between. The cost for a single person is usually about 30 euro with further discounts depending upon the number of people in your party. It is important to make reservations with a shuttle service prior to departing on your trip. Make sure that you have a firm agreement on the price before you contract with a private shuttle company. Shuttles are good options for people with heavy luggage and/or limited mobility.
RER Train
This is probably the cheapest, fastest, and most reliable option. It runs about every 15 minutes from 5 a.m. until about midnight. It is 8.20 euro (5.80 for children 4-10). The express trip is about 22 to 30 minutes, depending which location in Paris you are stopping.
Route: CDG Terminal 2->CDG Terminal 1-> Gare du Nord-> Châtelet-Les Halles->St. Michel-Notre Dame->Luxemborg->Port Royal->Denfert-Rochereau
When you arrive at CDG, take the 10 minute walk down to the direction of the “Gare” (train station). There is a shuttle bus but it isn’t worth waiting for and dragging your luggage on and off. Go downstairs and you will see a ticket office with a line of other tourists waiting. You will see other people purchasing tickets at the ticket kiosk, but unless you have a credit card with a computer chip in it (American credit cards don’t have these), you cannot buy your ticket at the kiosk. Purchase your one-way or return RER ticket to Paris. The ticket will give you a ride to the RER terminal in Paris and a metro ride (if necessary) to your destination. Hold onto the RER ticket, you will need it to exit the station.
The positives of taking the RER is that it is cheap, reliable, quick, and it will get you close to your destination in Paris. The downside is that there is some walking and elevators/stairs involved. If you have lots of heavy luggage, it is not a good option. Sometimes on the RER you will encounter beggars and buskers.