There is evidence that adherents to the Atkins Diet can reduce their weight and keep it off, provided they stick to the diet.
One widely cited New England Journal of Medicine study from 2003 looked at 63 participants over one year, and found that Atkins Dieters lost weight faster initially, but slower later on. At one year, there was no significant difference between the weight loss of Atkins Dieters and conventional dieters. The authors noted that "adherence was poor and attrition was high in both groups" and called for longer and larger studies. It is worth noting, as the authors do in their Results section, that the Atkins group saw increases in HDL cholesterol (the good kind) and decreases in triglycerides (low triglyceride count is good) over most of the study.
Another study from 2003 in the Journal of Perception and Motor Skills showed that Atkins Dieters "will experience more fatigue, more negative affect, and less positive affect in response to exercise than those individuals who are not restricting carbohydrates."
These studies and a small number of other studies are discussed in a 2009 overview of the Atkin's Diet from the American Academy of Family Physicians. They conclude that the Atkins Diet can be effective, and while there are concerns about safety (most notably the risk of long-term heart disease), they assert that the evidence so far is inconclusive on that point.
The evidence that the diet is effective should not be taken as evidence that the theories behind the Atkins diet are correct, however. One researcher opined, in a WebMD article that, "No one has shown, in any studies, that anything magical is going on with Atkins other than calorie restriction. The diet is very prescriptive, very restrictive, and limits half of the foods we normally eat. ... In the end it's not fat, it's not protein, it's not carbs, it's calories. You can lose weight on anything that helps you to eat less, but that doesn't mean it's good for you."
The high attrition and low adherence mentioned in the first study also tends to support the widely held view that highly restrictive diets, like Atkins and many other diets, are difficult to maintain over the long term. Thus, while they may be effective in the short term at reducing weight, standard lifestyle changes like eating a balanced diet and exercising are likely to be superior for life-long weight loss and health.