I think it can be a great way to immerse yourself in learning a language. At the same time, you are definitely an employee of thr family and you are there to work as a full-time 40+ hour a week babysitter. 

Living with a family you work for can be really really hard if you have ever lived alone...you xan have rules like curfews, not having friends over, what you can and can't eat from their fridge, etc. It can also be hard toset boundaries since you are living in the family's home and you can wind up working a lot of extra hours...and even if you don't wind up working more than 40hrs each week, most families have less than ideal schedules--ie, 6-730, then 14:30-18-30. Mon - Fri + a date night on the weekends so it feels like you are never free.

The other thing that xan be really hard is if thry don't live in the city and don't give you a car you can use...you wind up really isolated.

There was a Spanish au pair girl at my langauge school in Rome. I think she must have had a contract that had her working less than 40 hours and allowed her to go take classes at the language school. That approach seems more useful to me than just living with the family, because how much you learn there depends very much on the parents.

I'm not great with kids so it was never a good idea for me. I think you should choose the organization very carefully: will they help you if you don't get along with the family and reassign you to a different one? Some people think it's better/cheaper to cut out the middle-man and get hired directly by a family. Personally I would think that that's riskier because if there are problems you don't have anyone to help you. That's what the organization should be for, and that's why I'd take great care choosing one.

Katherine K, I was reflecting about this possibility but you made me afraid :p

Yesterday I watched some videos in YouTube of girls who have done this experience and a lot of them were really happy of it?

Have you experienced it?