I'm sure the ladies can chime in with their personal experiences but I had a co-worker years ago who did
Shake Weight.
She was a hard core fitness buff. You name it, she did it. I remember when she first took it up she came in one day all stiff and sore. Said it was one of the best workouts she had ever had and she was sore in plces she never even knew she had muscles.
It's probably alot like Yoga or Tai Chi. Looks easy and alot of people think "WTF! That's not a work out." Until they try it.
That said I still thnk the #1 workout for the ladies here is still the SHAKE WEIGHT!
http://www.youtube.com/watch?v=xXHUdvvHTkw
http://www.youtube.com/watch?v=ECFkHkhYMcE
In fact all the ladies should be submitting videos of their Shake Weight workouts.