My thoughts were the same as whitewave's, i.e. what are their terms? Would they guarantee achieving a minimum price for a given fee or would the fee be considerably decreased if you received a lower offer? I'd also do some research to see if there are any articles about the subject or users' reviews.
I've never heard of anyone using them - I expect those that have very expensive properties, a good income and little time to "dress" their homes to make them more attractive are the most likely to use such a service.
Also, presumably you would need to get several valuations from other estate agents to get some idea of what is a "good" price, a "realistic" price, a "quick sale" price, etc. etc.