Been to Mechant Boeuf a few times for birthday celebrations, and for small gatherings. So I have seen the service for both very large groups (25 people!?!) and for small (4 people).
The place is always packed, so its important to call ahead.
The service has been mostly adequate with a couple mishaps that weren't too obtrusive to the overall dining experience.
The food is not bad...I have always gotten the Mechant Burger (hope you like gruyere) when I go and it usually doesn't disappoint.
The atmosphere is very trendy and the crowd usually reflects that. There is a DJ who plays an eclectic mix of music (he is sporting a gigantic beard now).
It's a great place to celebrate something or to have a romantic meal.