Wow, I'm astonished that I haven't rated Léon de Bruxelles yet :O This is only my second time, though.
I'd give 5 stars for the food, but 2 stars for the service (at this location, at least). I enjoyed my experience here both times, but my food and my company were definitely the redeeming factors. Good things first, because it's more likely that you'll read those :p
PROS
+ It's a very large restaurant, so even though it's pretty popular, you're very likely to get a seat. And it's good for large groups and children.
+ They're open every day, including Sunday evening! And you even get mussels à volonté (unlimited) on Sunday evening. What a steal :O
+ There are a ton of choices for mussels and beer. I've never gotten the beer, but the mussels are great. I especially like the moules à la méridonale (mussels smothered in tomato sauce, cheese and pesto, mmm!).
+ They have an English menu, and speak English fine, if you need it.
+ Portions are very generous. If that one plate of moules à la méridonale isn't enough, you can get another plate for only 2€! Incredible.
+ Unlimited fries! And depending what time/day of the week you come, you can get other unlimited goodies. :D
CONS
+ Bad service #1: I came here for Christmas with two friends. One of them didn't speak French, so we spoke in English and got the English menus. The waiter put these red ribbons around our wrists, and when we asked him what it was for, he mumbled incoherently something about Christmas. Okay, cool ... However, at the end of the meal, we found out that we each got billed 1€ for the charity bracelet. I'm definitely not opposed to charity (and it was only 1€), but I would've very much appreciated it if he had told us what it was for. I was shocked by this behavior because I've only heard of it as a tourist scam in the touristy areas of Paris.
+ Bad service #2: My friend got moules à la cocotte (steamed in wine) and I got mine à la meridonale. His came steaming hot, but to my surprise, mine was cold. It wasn't cold as in just-came-out-of-the-fridge cold, but the cheese had congealed and it just didn't taste very appetizing. I was seriously displeased. After four mussels, I couldn't stand it anymore and broke through my normal politeness to ask them to reheat it (please). It came back a lot better.
+ The Belgian waffle was pretty disappointing. Nothing like a real Belgian waffle, and really nothing spectacular at all. In fact, it was overcooked and tasted slightly burnt.
Overall, I'd recommend checking the site (you can get 10€ off for dinner if you go with at least one person) which will tell you which days have good deals. Sunday night should be excellent :) Don't bother with the formule; the mussels are filling enough. If you really want dessert, get one from Pâtisserie de Rêves nearby or something.
If not for the bad service, I would really, really like Léon de Bruxelles. Perhaps I should try another branch. After all, unlimited, delicious fries? You're halfway to my heart already. read more