If it's your first visit, here's a bit of advice: in the vestibule, before you enter the store, take a number. This is the same procedure whether you are just there for a loaf of bread or are going to stay and have coffee and a croissant. Knowing your numbers from 1-99 in French will help you get served. If not, look up at the red glowy number behind the counter to the left.
Kind of an eclectic atmosphere in the seating upstairs. Bonus points for having a coffee-doctoring station (milk, cream, sugar, napkins, ...) upstairs so that you don't have to run back downstairs with your cup of tea or java.
Pastries are okay. For my money, Olive & Gourmando or Première Moisson are much better. Coffee, meh.