As far as I know, they're safe to use in reasonable amounts. I've never heard any kind of research linking them to cancer or anything like that.
I love sugar free syrup and sugar free peanut butter cups. However, be aware that they can cause gas if eaten in excess. Really, really, really bad gas and for some people diarrhea. This is because most of it can't be broken down by your body, but the bacteria in your digestive tract can break it down.
They also don't contribute to tooth decay like sugar does, so that's a plus!