I've been doing less than 30 carbs since July.
I have gone from wearing size 38 to size 34 (down 3 sizes) and the 34s are getting baggy on me.
I am sure I am eating many more calories than I was eating on the low fat/low calorie diet I was on prior. I wasn't losing weight on the low fat diet, even at 1600 calories.
The science is that by restricting carbs, your body gets into a metabolic state called Ketosis. The diet coerces your liver to create high concentration of ketones in the blood and low levels of insulin. The idea of the diet is to starve the liver of glycogen to cause it to create the ketones. While in glycolysis, the body generates high amounts of insulin which converts carbs to body fat. While in ketosis, the fat reserves are burned for energy.
The diet is not just the reduction (not elimination) of carbs, it is the change of your diet from traditional most of your calories from carbs to one that is most of your calories from FAT. You will not lose weight if you do not eat enough fat. This is contrary to the low fat/low calorie diet school of thinking.
While in ketosis, you don't have to exercise to burn calories, count calories, etc. This kind of diet is often called a "sedentary diet" as you burn fat while doing nothing. Ketosis happens naturally overnight since you aren't eating carbs while you sleep... so you wake up weighing less than when you went to bed.
That said, I have been walking at least 30 miles a week and running maybe another 3-5. My weight loss has been steady, but not as dramatic as some claim. I think it's because I'm adding a lot of muscle at the same time, and muscle is dense (weighs a lot). That my waste has reduced by 3 sizes is a pretty good indication the diet is working incredibly well.
The notion that you somehow stop doing low carb when you hit your target weight is a bad one. Most people I know (from reading keto-diet blogs and message boards, etc.) simply see it as a lifestyle choice and don't miss non-keto friendly foods. I know I don't miss them, and I don't expect to stop eating low carb ever. As I've gradually migrated my kitchen from high carb foods and condiments to low carb ones, I am not suffering for food choices. I've found bread substitutes, pancakes, fax meal muffins, etc., that make me plenty happy. The challenge of creating recipes that are low carb is part of the fun of it all, as well.
I will end up changing the meaning of "low carb" for my meals. When I want to stop losing weight and maintain my target weight, I will gradually increase the number of carbs I eat a day. Right now I'm eating < 30 (it is REALLY hard to eat < 20). I'll probably end up eating as many as 100g per day. Anything less than 150g is considered "low carb" by dieticians/nutritionists.