Breast feeding and weight loss after pregnancy

I've had several recent conversations with moms of young children about the topic of breast feeding and weight loss. Since breast fed infants are provided about 600 calories through breast milk, logically, breast feeding would help with weight loss in the mother if she doesn't increase her intake by as much. It's also a notion supported by scientific research as well as nutrition guidelines for lactating women, who are told to consume 500 calories more per day than non lactating women. In my own experience, breast feeding helped to take off the weight I gained during both of my pregnancies.
But, recently two women I spoke with on separate occasions, told me that for them, breast feeding actually kept the pounds on and they didn't lose the weight until they stopped nursing at about a year. I wonder if psychologically they continued to "eat for two" until they stopped nursing or if their body adapted to the increased needs and "hung on to the weight" during this time? Have you had this same experience? I'm curious how many people have a tough time losing weight while breast feeding - perhaps because of a stoked appetite - but then take off the weight once they wean.