My friend took me there for the first time 4.5 years ago, and I remember my experience there quite pleasant. My friend had the foie gras, and it went very well with bread.
4.5 years later, I find myself back at L'Express again, and my 2nd experience remained pleasant. We didn't have reservations, so we did have to wait 45 minutes to get a table (we walked around rue St. Denis, so no problem). The decor is reminiscent of an European (presumably French, duh?) bistro: very brightly lit and simple tables with a giant mirror.
The food was decent. I love the little pickles they provide in the jar. I got the octopus with lentil salad. The plating was very interesting (octopus packed like a can with the lentils inside), and the salad was tasty. My friend got the warm goat cheese salad that she said was delicious, and another friend got grilled salmon with spinach which the salmon was big. We then got 3 desserts that were all very tasty and light to eat: apple/pear pudding, the orange 'flan,' and a chocolate mousse cake. My small concern about this place is the service is a little slow and we did have to wait a while to get a table.
So I can say that in that 4.5 years span, L'Express remains consistent with the quality of the food (and unfortunately with the wait as well).