A December 2006 piece quotes ACSH's Dr. Ruth Kava on sugar substitutes:
"Anything that helps you cut calories can help with weight loss. But the extent to which it is helpful depends on current practices. If you drink three to four sugar sodas per day, switching to sugar-free sodas can be substantial benefit. But it may not be if you drink one a month," says Ruth Kava, Ph.D., R.D., director of nutrition for the American Council on Science and Health, who coauthored a review of safety issues for sugar substitutes in the 2006 Comprehensive Reviews in Food Science and Food Safety...
"I don't think safety is a big issue. With the exception of phenylketonuria, sugar substitutes are safe dietary ingredients," says Kava, who notes that many sugar substitutes are so sweet that we consume tiny amounts...
The hidden danger of sugar substitutes is thinking that if you drink a diet soda for lunch, you've saved enough calories to splurge on a hot fudge sundae. Just because a food product is sugar-free, it doesn't mean it is low-calorie. Keep an eye on consumption. "Sugar substitutes aren't necessary; you don't have to use them. Sugar isn't so awful. It's a matter of moderation," says Kava.
See also: ACSH's report on Sugar Substitutes and Your Health.
