I simply love Munich and think Munich airport is one of the best airports I have been to. It is so clean and spacious.
Everytime I arrive in the airport, I get the impression that it is one of the least busy airports, but apparently it is one of the busiest airports in Germany.
From airport to Munich city centre, there are two S-Bahn's running (S1 and S8) regularly and usually take about 40 minutes to get to the centre.
Every 20 min there's a bus, which is run by Lufthansa and takes 25-35 min from Hauptbahnhof (main train station) in the centre to the two terminals at the airport and back. A round-ticket costs €15, one-way €9,50 apiece.
The S-Bahn is very convenient: the airport is served by two different lines, the S1 and the S8, which link the city with its hub every ten minutes. It takes 40 min approximately to Marienplatz or main train station and is pretty well-priced, especially when you aren't alone. Up to five adults and as much family members of one adult which under the age of 14 can use one "Partner-Tages-Karte" (a "partner" day ticket) but pay only €16 for the ticket. And the best is: you can use the ticket the rest of the whole day throughout the city.
Homepage of the Munich Metro Transport Authority (english)
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