FESTIVE. Every time I went to this place I had the time of my life. It's like stepping into South America with a bar full of French and international folk. The place is Brazilian and glows in the warm yellow and orange colors.
They have original sounding dance music - Latin-flavored or electronica, great mojitos, great dancing, and a great social scene. The only con, which is simultaneously a pro, is that it gets so unbelievably hot in there that you will be soaked in your own sweat. Then shortly thereafter you will be soaked by the bartenders who often turn the hose on people. Dress in layers so you can peel clothes off as the night goes on. It's worth it though to feel like you're in an indoor Carneval.
The place gets packed weekend nights and there may be a long line to get in. You can beat the line by having dinner there. They do have dinner; they serve Brazilian fare and it's pretty good. Seating is on picnic style tables adding to the festive environment. It's a good way to start the evening there.
Metro - Republique