Hype vs. Facts
"The Medical Letter", which many physicians rely on for prescribing recommendations, has published issues on aspartame (Equal), sacharrin (Sweet and Low), and sucralose (Splenda). Aspartame is the safest and most tested. Sacharrin causes bladder cancer in animal studies (and has been banned in Canada). Sucralose may cause immunologic (body's defense against infections and cancer) and neurologic (nerve) problems.
A few folks, in addition to phenylketonurics, do have trouble with aspartame with headaches and allergic reactions. I recently read of it causing joint pain. It's still the safest for most people. In fact, a recent review of the scientific evidence on aspartame by the American Dietetic Association found the following: Aspartame does not increase appetite. Aspartame does not cause weight gain and may be associated with weight loss. "Aspartame consumption is not associated with adverse effects in the general population."
Another sweetener that gets no attention has found its way into many drinks, yogurt, and snacks. That sweetener is acesulfame, which has been linked to cancer. Whenever I see it on a nutrition label, I call the consumer hotline and complain. Using this less tested potential carcinogen to replace the safe aspartame is a PR stunt because no one knows about acesulfame and it has not been the victim of a media witch hunt like aspartame has.
Erythritol (Stevia) has not been tested, so may or may not be safe. It's natural, but so is arsenic. 11/22/2013 update: Truvia has been tested and seems safe. Other Stevia supplements may contain chemicals that impair fertility and cause birth defects. (Mother Jones published online today.)