This women-only local chain of fitness centers is beyond popular, if the numbers mean anything: 15,000 members, 500-plus employees, and four locations (plus two non-profit fitness centers). Over 30 years, clearly Healthworks Fitness Center has figured out what women want in the fitness world. The Back Bay location in Copley Square is steps from the nearby subway station, making it convenient for those coming to work in the area, as well as residents of the Back Bay and the South End. The formula is easy: offer literally hundreds of group classes each week, a team of personal trainers on tap and provide a spa-like atmosphere in the locker rooms, which are stocked with amenities like hair dryers and towels. A wide variety of equipment, plus services like childcare, nutrition counseling, massage therapy and special events and programs, keep things buzzing all day long.