We have had au-pairs for 7 years now, and are up to three kids. We've had great experiences with all of the au-pairs and highly recommend the program to everyone. The cost is reasonable, and the childcare is fabulous.
All of our au-pairs have been from Germany, so we have had them all speak German with the kids for a true intercultural exchange program. In addition the au-pair is responsible for making the kids clean their rooms, helping fold and put away laundry, driving kids to and from activities, and generally being a third adult in the household. I wouldn't trade my au-pair for anything.