Yikes! I'm astounded by some of these rude comments about Au Pairs and the families that host them.
We have a fabulous Au Pair from Germany, we've met many of the other girls who came over through the same agency. They're, for the most part, treated wonderfully by their host families. Of course, some girls that came over with our Au Pair were not a good match with their host family (the girls all stay in contact with each other) and are then placed into a different family by the agency.
If you go through a reputable agency, you are allowed to choose the family you stay with. We had to provide tons of information about ourselves, have a formal interview, have her room inspected to make sure it was appropriate, etc. The Au Pair receives support from a local coordinator, has monthly meetings, is well-trained (attends a training school), etc.
I resent the implication that families that host Au Pairs are weird. Many families in my area have au pairs - it's a wonderful way to have an extra pair of hands around the house. It's like having a daughter/big sister.
Our Au Pair is taking college classes (at our expense), drives a 2004 Volvo S80 (at our expense), has traveled with us on all vacations (at our expense), has made tons of friends in the area, travels on her own on her weekend off (this weekend she's heading to Washington, D.C.).
In any event - being and having an Au Pair is a wonderful, cultural exchange when you go through a top-rated agency and choose a family that you feel 100% comfortable with.
I am sure there are many families that treat these girls (and guys - lots of guys are Au Pairs as well) terribly. That's why the agency you use MUST be top of the line and have lots of support in place if the placement doesn't work out. Just like you can end up with an awful college roommate - you can also end up with an awful family. It's how easy it is to get out of that situation that really matters. It's all about the agency.
Good luck to you in whatever you choose.