Acesulfame K is another sweetener, with a similar level of sweetness to aspartame, compared to sucrose ( about 200 times sweeter).
Nowadays, you will often find a blend of 2 sweeteners in fizzy drinks, simply because it would appear through taste and preference tests that the blend of sweeteners can cancel out that "artificial" or bitter aftertaste that some people have reported with sweeteners.
As to safety - There have been many safety tests,mostly animal studies, conducted over the decades since it was first found, and all conclude it is safe with no immediate or long term adverse risks. There has been 1 study where it was claimed that it might be a carcinogen, but that claim did not stand scrutiny.
Bottom line - I have every confidence that such sweeteners are perfectly safe, unless you have PKU (aspartame), or are drinking 100s of cans of fizzy drinks per day :)
http://ec.europa.eu/food/fs/sc/scf/out52_en.pdf