My dad is a biochemist and won't touch anything with aspartame in it. He won't touch Splenda, either. I figure, if a biochemist won't go near it, then it must be bad stuff. My dad says that the regular stuff is way better for the body, even though it's loaded for sugar.
What's interesting is that I've actually read accounts and done research that indicated that diet sodas can CAUSE weight gain. And, when you look at how weight has skyrocketed in the years since sugar-free this, sugar-free that, and diet sodas were invented, I don't think many can say that these items really contribute much to weight loss.
Plus, caffeine is a problem for people trying to lose weight, as someone already mentioned. I stopped drinking caffeine and lost something like 3-5 pounds in two weeks. All I did was stop the caffeine-intake...and I lost that much weight.