This is a topic I cover in my book, The Asian Diet: Simple secrets for eating right, losing weight, and being well (theasiandiet.com).
Caloric science in inherently flawed, and I can't believe no one else has pointed this out yet. The way that they measure calories is: they set it on fire in a device called a Calorimeter. They rationalize that the amount of energy liberated by combustion in the lab is the same amount of energy that would be released when the food is broken down enzymatically in the stomach. Now, those are two very different processes and environment, so I'm not sure that this assumption is correct. But,even if it is true...let's assume a muffin has 200 calories of energy in it. When I eat that, I put 200 calories into my body, but I don't absorb all of it. Some of it passes through. Our true caloric gain or loss is determined by: how much we put in, minus how much we burn, minus how much we excrete. If you have dysentery, it doesn't matter how many calories you consume, you will not absorb much of them. Now I'm not suggesting that we all start burning our feces and measuring their caloric value, I am just saying that the number of calories ingested is not the whole story. The average person in China consumes between 25-40% more calories than the average American on a daily basis. Even the sedentary office workers eat more calories and have less obesity than do we.
It is not the number of calories that is important. It is the kinds of foods and how they are prepared and ingested that matter. What you want is efficient digestion of good quality, natural food.
Calorie-free just means that it is a substance that we cannot absorb. This does not mean that it is benign. There are things that we could eat that we would not absorb but that could still be harmful. Take for example a razor blade, or a marble made of plutonium. I am not saying that Aspartame, Splenda, Saccharine, etc will cut you or give you radiation poisoning. My point is that just because something is not absorbed as it passes through our bodies does not mean that it does not interact with and affect the other cells and molecules with which it comes into contact. I would rather have a snack with calories made with a natural sweetener rather than a calorie-free option made with unnatural ingredients.