Ha, same OP - I had friends rave about it, so we tried it. I absolutely hated having an au pair. I learned a lot about myself during that period - I had no idea I was SUCH an introvert, I like my own space, I am very private. We paired with a lovely young woman but she was the complete opposite of me - wanted to do EVERYTHING together, nosey (but in an innocent way... like if I came home with Nordstrom bags, she'd be like, ooooh! What did ya get???), very social, very talkative. Yes, you can try to get someone with a personality similar to your own, but I also just didn't know until we tried it how much I'd dislike it. We did it 2 years (an AP can stay up to 2 years - even though I was pretty done by the end of Year 1, she did basically become like family so we did a second year too... but it was "family" like a clingy younger sister or niece who you love, but you also just want them to move out and leave you alone). We keep in touch. She's a sweetheart, the kids liked her but man I didn't like hosting someone. I had enough on my plate, then to add in someone who looks to you for social interaction, understanding our country, living in America, homesickness, hosting her birthday party and hosting her friends, including her in Christmas events and presents and stockings, including her in vacations, like getting her ski rentals when she'd never skied, surf lessons, etc. It is A LOT of time, energy and money to host someone!
Someone said above it's the cheap option - obviously they've never done it! It is "cheap" on paper but anything but in real life. Money was not our issue. We got it for the convenience piece, and to be fair, it was incredibly convenient. Top tier. Take her anywhere with you. No more snow day cancellations. No more car breakdowns, or "my dog is sick." Super consistent, plus you can change her schedule weekly - just give her a heads up. But no, not cheap - you'll need to include her in meals, going out to eat, vacations, your phone plan, and of course give her a car. And just like so many other APs, ours had an accident and our insurance went up quite a bit.
I guess it could be cheap if you lived a really frugal lifestyle, but we travel a bunch and eat out a bunch and splurge sometimes, so it wasn't any cheaper than a nanny for us.
I have friends who did the program for years and loved it, so it just comes down to personalities I guess.