Mechant Boeuf is one of those resto-bars that will morph into whatever you like. If you're in jeans and a sweater, you can treat it as a place to get a tasty burger and chill. If you're in a suit, the decor gets outright glitzy near the bar and far wall so you feel like you're in a trendy supper club. On a date, with friends, whatever, Mechant Boeuf seems to fit the bill.
The service is typically average, and the food is actually really good - I've been a dozen times and always found it to be delicious, albeit not the most adventurous menu. In any case, I don't think the point was to experiment too much, just get the basics across (good burgers etc) without too much fuss in a beautiful setting. Works well!