I personally think Atkins is gross - it doesn't really teach you much about healthy eating and the side effects (from what I've heard) are nasty - bad breath, gas, etc. You would be much better off doing South Beach or CSIRO as I understand both focus on healthy protein, low GI carbs and good fats. Dukan sounds similar to these from what I've read but I don't know it first hand nor do I know anyone who has tried it.
If followed properly it is really very much like South Beach and CSIRO. Most people think they stay on the first 2 weeks the whole time and it does not work that way. Basically you end up swapping bread for salad and pasta for veg. Just like any other way of eating you can twist it to be unhealthy. I am sure there are those on WW and similar diets who rely a lot on the prepackaged food, rather than cooking fresh.