I worked for about a month as an au pair and it didn't work out. My advice is to get to know the family well before you go. I think you could have a great experience as long as you know what is expected and know the kind of family you are going to live with. Here is my experience:
I had 2 job offers, one in France and one in Martinique, a French island in the Caribbean. On the surface, the Martinique job seemed better, only 2 kids (there were 3 in France), more money, tropical island. But if I would have paid attention, I probably could have figured out which would be better.
I emailed the woman in Martinique and spoke with her on the phone, in English, for about 5 minutes and then she offered me the job. Clearly, she wasn't too concerned about sending her kids off with a stranger. The family in France emailed me a lot, asked for references and conducted a lengthy phone interview in French and English.
Hindsight is always 20/20 I know, but if I would have looked at the messages each family was sending, it is obvious that the family in France was more concerned with their children's well-being and would have probably been a good, loving family. However, I was a drunk college kid and chose to go to the tropical island. Probably one of my only/biggest regrets in not taking the job in France. I bet it would have been good.
Okay sorry, back to the topic. Just be sure to get to know the family before making a commitment. Give them references, ask for their references. The best would be the contact information if that have had an au pair before.
Hope this helps!