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· Roissybus: If you're just dying to visit the Opera Garnier, why not do it straight from the airport? The Roissybus departs from the ground transportation area at each terminal, and goes direct to the opera house, from which you are able to hop the Metro or walk a short distance to Place Vendome and the Hotel Ritz for a little more immediate sightseeing.
Total cost: €8.90 per person, each way
Total time: 60-75 minutes, dependent on traffic

· Air France Shuttles: You don't need to have flown on Air France in order to utilize these shuttles. Leaving from Terminal 2 (take a free transfer from Terminal 1 to reach it), there are two coaches to choose from, depends on your destination: Line 2 to Porte Maillot and the Arc de Triomphe, and Line 4 to the Gare de Lyon and Montparnasse.
Total cost: €14 per person, each way; €22 round-trip
Total time: 45-50 minutes, dependent on traffic

· Taxis/Towncars/Limos: Always the most private and comfortable mode of transport into the city, taking a taxi or car means you'll have to interact with the locals right away, so practice your French for "are you taking me the long way?" Taxis can be gotten at the usual taxi queue, while it's better to arrange towncars and limosines beforehand so that the price and destination are settled.
Total cost: €50 flat fee for a taxi, and €105 and up for limos
Total time: 60-75 minutes, dependent on traffic

Related Stories:
· Paris Travel Guide [WikiTravel]
· Paris Travel Coverage [Jaunted]
· Airports Coverage [Jaunted]