Xylitol is controversial because it is a highly refined 'artificial' sugar- it's an alcohol sugar which will give you diarrhea if you consume a lot of it. Mercola has a good bit about it on his site. However, in theraputic doses, such as found in toothpaste and gum, I don't worry about it. To me, for that use, it's a small dose and you're spitting it out. It's a localized action on the teeth. The studies who have shown benefit to the teeth from consuming it have used small amounts, like 7g spread out throughout the day.
So we use it in toothpaste and gum and mints for those in the house who have decay, but we don't cook with it.