Yes, it's visited by many tourists - but it's also visited by many locals and considered to be a city institution. When I'm in Brussels I try to make a point of visiting Chez Léon at least once, just to grab huge pot of traditional Belgian moules (either with white wine sauce or green curry), a bottomless bowl of expertly-salted frites, washing it all down with a cheap local beer.
Having been in business for 120 years, these guys know what they're doing when it comes to mussels (I suggest the moules provençal). While it is surrounded by sub-par restaurants that merely survive by scamming tourists into their restaurants by dragging them in off the street, Chez Léon is a diamond in the rough. Fantastic if you're on a budget, it'll more than give you your fill of moules et frites to power you through an afternoon of exploring.