Take the 4-star rating with a grain of salt; the lobby/reception area is lovely as are the bathrooms, the bedrooms are another story. It's more of a 2 1/2 to 3-star hotel.
Try to avoid the 11th and 16th floor rooms; actually, if you receive a room you don't like do NOT hesitate to request another. Even though you bid thru Priceline don't accept a bad room (see link below for "reviews" and helpful info) - be POLITE but FIRM.
Appearantly there is some street construction going on in the area between the Balard station and the Sofitel.

Read this thread for further info:
Sofitel Porte de Serves Reviews/Info

As for breakfast, I would take the Line 8 to the Ecole-Militaire station and walk a block or so to rue Cler and either pick up breakfast in a patissarie or boulangerie or have it in a cafe there (will be less expensive and tastier).
My favorite boulangerie in rue Cler is about a block or less away from the Jeff de Bruges chocholatier. It's across the street from a "deli" (where they have really good picnic food) and has the best baguettes that you can purchase as half loaves and good quiches (sold in front of the boulangerie entrance).
Alternately, you could take the Metro further into central Paris and have breakfast there, I just suggest not eating at the hotel unless it's included in with your stay.

If you need more info or suggestions, PM me; however, I am off for Florence soon so may not be able to respond during mid to late January.

RxTravel