I've been on both lo fat and lo carb diets.
Of the 2, the lo fat is the most unhealthy I'd say. When I went to lo carb and put fat back in my diet, I couldn't believe how much better I felt. I think the low fat diet was slowly killing me.
OTOH, these days I do not do either diet, though I come closer to the lo carb.
Instead, I avoid eating too much processed food, trans fats, processed sugar, white flour, sodas, that kind of stuff, and watch it on the starches. With just a little exercise (15 minutes of running per day, 3-4 times a week) and not drinking too much, I stay pretty trim.
I think just avoiding the obvious eating sins can do most of the job for most of us.
You would think so, but I am constantly amazed by the sheer number of people who have been fat their entire lives that go on Atkins for a year, and lose upward of a hundred pounds! Many of them are forty and fifty year olds right here on freerepublic. These same people claim to be consuming five to seven THOUSAND calories a day!
I've got to believe weight is like cholesterol, very dependent on your particular physiology. I've known couples who eat the same food, weigh within ten pounds of each other, and have more than a hundred point spread between their cholesterol numbers.