I live in the Marais district in the 4th arrondissement, and one of the main reasons for that is the nearby presence of Rue des Rosiers.
Rue des Rosiers gets more and more full of pricy boutiques each month but it is still the heart of the Jewish community and the best place in town to eat falafel sandwiches.
There are two main shops on the street that compete face-to-face to entice you in for one of their sandwiches. If you believe famed vegetarian Lenny Kravitz and judge by the length of the lines the best shop is L’As du Fallafel. However, a blind taste test I just did reveals that the rival Mi-Va-Mi just edges them out. It also has the best frîtes.
PROS: Filling, fast, cheap and the street is great fun. Open Sundays
CONS: Lines can be long, not organic, schwarma sandwiches also sold. Closed Friday Evenings, Saturdays and Jewish Holidays.
UPDATE: My most recent taste test gave L’As the edge on sandwich quality over Mi-Va-Mi. But they are both great