Would you, in all honesty, rent your room to a lodger?
Our aupairs - we really have had a lot (13), but I was a SAHM for the first few years so happy to have them for only short periods - they are much keener in the first few months, and really do tail off quickly.
We have had one total disaster - Estonian girl who stayed for two nights and literally run off at dawn with the contents of my purse (fortunately not much).
We have had a Bulgarian girl who appeared to be a visa scammer - stayed one night and left due to my children's alleged headlice (they didn't, and I know because I checked them the day before she arrived).
We had a lovely English girl who managed a week, but broke her leg on a home visit and obviously couldn't come back to us. We thought that was going to be a great arrangement up until then.
We had a German girl who really wasn't cut out for aupairing. She seemed too good for our domestic tasks. We have also had 3 girls who got really homesick and only stayed a few months each. They were really torn, especially the last one like that who seemed to have problems sticking with things.
Our first aupair, we made lots of mistakes. She was nice, we are nice, and we got off to a good start. Her English was poor and we weren't paying enough - the stress took its toll. We have learn't since then.
We've have five other aupairs that have been good. They've stayed the course and enjoyed themselves here.
We've only had on really bad experience - the Estonian - and one pretty bad - the Bulgarian. After the Estonian, I had about 10 days to find a new aupair and managed to get one with two days to spare. After the Bulgarian, I had help from frieds and DH, and then the brief English aupair, then two weeks of an after-school childminder. The long term aupair started 1st October, so it was, in reality, 3-4 weeks of turmoil.
It is not a stress-free route. You really don't want them in your house when you are on holiday, so it's good to get them to arrive just a few days before you need them. But if it goes belly up, you have very little time to sort out alternatives.