I worked as an au pair in Beijing for 9 months, and I found it to be an immensely rewarding experience. I learned so much about Chinese culture, and the agency provides free Chinese lessons! Through their help, I aced my HSK 4 exam. The staff are very lovely, as well, and always willing to help.
I would honestly say the agency is the best part of the experience. Unfortunately, being an au pair is not always easy. If you plan to apply, be aware that your host family may not always follow the rules set in place by the agency. You may work more than the promised 25 hours a week, or you may be asked to do things you're uncomfortable with.
Beyond that, this program was amazing, and cost nothing. I actually came home with more money than I started with, and even better, I came home with more experience and knowledge.