The problem with everyone's view of Atkins is that they only focus on the Induction period of the diet. There is much more to it then that. There are actually three phases. You gradually add back carbs and portion size until you are no longer losing, just maintaining your weight. The latest book, Atkins for Life explains how to do it and has hundreds of recipes.
If you stop following the concept and go back to old eating habits, of course you'll gain weight back. Conversely, if you exploit/abuse the induction period as a quick way to lose weight, without doctor supervision, your body will feel the effects. The induction period is only meant to be strictly followed for 2 weeks. Then you merge into a slightly higher level of carbs, protein and fats until you are within 10 pounds of your goal and then merge to an even higher level of carbs to slow down your weight loss, adding more carbs back in until you reach the point of maintenance.
I'm not strictly following Atkins but have drastically reduced the amount of white flour, sugar and carbs in my diet and can't believe the difference in the way I feel. You can avoid the constipation by picking specific foods to eat and by ensuring your intake of water is up to par and that you have a sufficient level of fats in your diet. Almonds, broccoli, cauliflower, carrots, lettuce, spinach, coleslaw, avocado, chicken, salmon, ground steak, porkchops, eggs, cheese, yogurt, real butter, oatmeal, and berries of all kinds are just some of the foods I'm eating. Heh, there are some days when constipation is the last word I'd use to describe my bodily functions.