if splenda caused a subset of people to be violently ill (like they are claiming) it would be off the market so fast. i find it hard to believe it's because of 'allergy/sensitivity'.
Additionally, everything I've read regarding it shows that it doesn't cause people to be ill like this. The only sugar substitute I know to have an effect like that is stuff like sorbitol, and it doesn't cause people to vomit. It can cause diarrhea if consumed in significant quantities though.