I almost hate to review this place, because so many people have given it a 5-star rating, and my experience was definitely less than par.
I'll keep it short: people clearly pay for the ambience (i.e., wonderfully traditional French bistro, where you can linger for hours, drinking wine and/or coffee, and enjoy some good food.) The meal was okay (steak frites), but I've had better for less.
If you're in town for a few days and want to give it a go, shoot for mid-to-late afternoon in between the lunch & dinner rush, and you may be able to get in without a reservation. If you're aiming for a dinner seating, book ahead a few days (at least).