I have had an au-pair and am just about to get another one, largely because I have a flexible (working from home) job and can't afford any other flexible option.
My experience with my last au-pair was largely positive. I'd say that you need to be very tolerant of someone in your house (if you're not, don't even consider it), and also, you need to remember that these are very young girls, who are doing the job because they want to learn English, not because they want a career in childcare (although some of them do). So they need a lot more guidance than the average professional childminder. However, in some ways, that can make it easier, because they don't have pre-conceived notions of how to do it, so they do it your way, rather than theirs.
Having said that, my au-pair was a very constructive, pro-active person, and she really got to know the children well - she made suggestions about changing DD's nap times, for example, and things like that, before I had thought of it (maybe I'm just a crap mum!) If you get someone who comes from Eastern Europe, they tend to be much more mature and used to looking after children, they don't find them frightening or alien.
I was very careful to make sure that the au-pair didn't do more than 25 hours until I knew her really well and trusted her (and then paid her more for it), because I think it is a tough job looking after kids, and you need to make sure that you don't load someone so young and inexperienced with too much responsibility.
I had one downside - my au-pair bonded incredibly well with DD, but not with DS, which became more and more of an issue, and in the end, was the major reason why I got rid of her. In every other way she was perfect, but in the end, I couldn't inflict a carer on my DS who actually lived in his house and whom he didn't like. The problem there was that she didn't like him at first, he picked up on it, started to dislike her, then when she changed her behaviour to him, it was too late - he just didn't like her, and after about four months of her being consistently nice to him, it just became obvious that he was never going to like her.
In retrospect, it was her own immaturity in showing her dislike of him that was the problem, coupled with my lack of understanding of her inexperience - and my lack of experience in having an au-pair (she was my first one), and lack of guidance for her - I didn't realise how big a problem it was until it was too late, and I was so pleased that she had bonded so well with DD, that I behaved like an ostrich with the DS issue. You live and learn.
And no, you don't have to give up your nice bedroom! Most au-pairs would rather have a small crap room, bad house, lousy furniture and mendicant single mother, than an unwelcoming, unfriendly family. They are young, new to the job, in a foreign country, far from friends and family, unable to speak the lingo (at first), and they need a lot of support and understanding. They want to be welcomed into the family, without being suffocated by it. It's a difficult balancing act, but it can be done, and it's very rewarding - people have been know to keep in touch with ex-au-pairs years afterwards.
If it helps, there are a number of web-sites which have chat-areas like this one, where parents can sound each other out about pitfalls. Being crap, I can't remember any of their names, but if you do a google search on au-pairs, you'll find lots.
Good luck whatever you decide...