I have used this website a number of times and yes I agree with the other review that it is time consuming and can be overwhelming finding an au pair through a website. However the website is really easy to use and you do get a big choice of au pairs from a wide selection of countries. It is very cheap to sign up for 3 months and I have found 3 really good au pairs!
We used au pair world to find an au pair as we didnt want to pay agency fees. It is time consuming going through all the applications but you just learn to be selective when choosing your short list ie want some from Spain, non smoking, experience with children etc. We had email chats and then used skype for a 'face to face' chat. Worth using if you have the time.