Unfortunately, there are very few long term studies out to substantiate any claim. But like any diet, if you go off it you will in all likelyhood gain your weight back. No matter what you do (low fat, low carb, surgery), you will gain your weight back if you don't watch what you eat.

In my opinion the diet that works the best is the one you feel you can stick with. My sister lost 75 pounds on a low fat diet, and has kept it off now for over 10 years. She made changes in her life that she can live with, and she is successful in sticking to those changes. If you feel you can stick to a low carb diet for life (!!!), you will in all likelyhood be able to keep it off.