Healthworks has 5 main locations in the Boston area, plus a couple of pilates studios and one spa.
I've belonged to a several other gyms before (Gold's, Bally and Club One in SF), and Healthworks is by far the best. Perhaps because it is focused on women entirely, it has nearly everything a woman might want in her workout or connected with it. There is of course the main floor with cardio and strengthening machines and weights, and then they have a couple of studios for spinning, step aerobics, yoga, pilates, dance, kickboxing and other classes. You can hire a personal trainer to structure your workout. From the locker room to the workout area, the place smells different than other gyms - it smells clean, as opposed to sweaty. The locker room has all the niceties you could think of to get showered and dress after your workout, and at least the Cambridge location has a hot tub, too.
The non-mainstream services range from towel service to pre-natal yoga to infant massage class to daycare to nutrition counseling.
The best part is that right alongside the really fit, you see women here - old, overweight, out of shape - that you wouldn't see at co-ed gyms. I really like that aspect.
I was on the student rate, which prevents you from attending during peak hours and which does not include things like free intro sessions with a trainer or the towel service (though I am not sure these are included with the normal rate either).