Surprisingly, Pub St. Germain (which is the largest pub in Paris), has a 24 hour restaurant that serves really delicious food.
This isn't your ordinary pub fare; there's salmon, steak, incredible soups, you name it. Even the cheeseburger is gourmet: no bun, rare, and smothered in Gruyere.
The service is decidedly snooty -- we had a waiter who made fun of our French -- but it was actually pretty funny, and they'll still be quick and efficient.
It's probably one of the best places in the city for a late night meal -- and the prices are reasonable.