Hi evangelina, whether an aupair is a help or a hinderance depends on the girl. If the arrangement works, you begin to wonder how you ever managed without her. if it doesn't, then you are really better off without.
You should have quite a good idea within the first few weeks. I have a 4 week probation period where either party can terminate on 1 week's notice. After all, the aupair might also decide we are not her cup of tea, in which case,
I would rather she leave earlier rather than later.
I do think that for it to work, you as the family need to have the right mindset to take the time to settle her in. 6 months is a decent amount of time. You will have to encourage your (older) dcs to go with her and her to play with them, show her the park etc. I have an aupair folder which has details of language schools, other aupairs to make friends with, pubs, cinemas, library, busroutes, transport info, local activities etc. I also have a detailed list of house rules and an hour-by-hour time table so she precisely knows what we expect.
And during the early days especially, to involve her in your life. Once she starts to make friends of her own, she will go out more and be less under your feet outside of working hours.
There is a certain amount of teenage issues to contend with. I don't particularly try to get stuck into boyfriend issues beyond a few questions from time to time. Other than that, my policy is to give her her privacy.
I also go for late teens to early twenties...
I do think that the right girl can be a great help if you take it in the right spirit and make it worthwhile for her as well. Otherwise yes, it is better to do without.