I did Atkins for a couple years and lost 100 lbs, but I also rode 5,000 miles on my bicycle and walked several hundred miles during the same period. Toward the end, I started tapering back on exercise and also saw a corresponding slow-down in weight loss. Of course, I was also getting closer to my goal weight.
I'd say exercise is key. Just doing Atkins alone might help you loose a few pounds, but exercise makes it go quicker and you feel better.
In retrospect, I would say that a well-balanced diet low in carbs -- but not as severe as Atkins -- plus a well rounded exercise program will give you the same weight loss and better health.