My DD is 16 now so I've almost got to the end of the line with hosting au pairs..
I've arranged it all myself a couple of times and have used agencies for 7 or 8 years.
As a virgin I went straight to DIY and found and hosted a couple of good candidates on Great Au Pair. On our third attempt though we were messed around by a girl who didn't turn up on the day she was supposed to. I used the site to try and find another person in a hurry but got fed up with people who seemed to be using it as a social networking site (mabe its improved since My Space and Facebook have taken off!!). In the end the delay in sorting my childcare out and the cost ended up being very expensive.
Since then I've used agencies.. The first was Glogovich and the woman there Laura was fantastic but I think she has sold up to another agency but if not then she is worth contacting 01883 624662
The Au Pair & Nanny Agency were pretty good and we usually try them. The last time we looked though they had stacks of short term candidates but not as many longer term girls.
For service the best agencies I have found have been London Au Pair and more recently we have been using 1st Choice Au Pairs
London Au Pair were really very good at supporting our au pair when she arrived, and kept in touch throughout her stay. They are always friendly and helpful if you want to ask for advice etc. Their registration process is not as streamlined as some but I have found that some of the slicker agencies don't seem have as much time for you so it's worth sticking to a good smaller agency once you have found one.
1st Choice Au Pairs travel to meet their candidates so when you talk to them you speak with the person who has met them in person. They have videos of each candidate so perfect to see how their English is. The girls we've had from them have been almost fluent in English. They also seem to go the extra mile in helping out with different requests.