Having lived here for 18 years and worked in restaurants for 7 years, I can tell you that the staff don't see a single centime of that 15% service charge. They get their salaries which are pretty low; anyone below the level of chef de partie, a line cook, gets the SMIC or minimum wage, currently around €9.50/hour for a standard 39 hour week. In smaller, non-chain restaurants you'll work 50 or 60 hours without overtime.
Many but not all people leave a tip, a few coins is the generally accepted amount; I always leave 10% because I know how much it's appreciated. I also ask that it be shared with the kitchen, although I've only worked in 1 restaurant where that happened at all.
My last job in 2010 was as second de cuisine/sous chef and my monthly take home pay was €1378 after taxes. That was with 7 years experience. The best salary I had in a restaurant was €1500 as chef in a startup restaurant which closed after 3 weeks. I didn't get paid.