FOS - a good or bad thing?

A client asked me about these saying they didn't contain sugar so were they ok? (in moderation of course!)http://www.therawchocolateshop.com/detail.asp?prodID=61&sort=9I looked up lucuma to find out it is a fruit of the Incas so that would explain where some of the sweetness would come from.But is FOS a good thing or not?ThanksEmma