I'd imagine you need to start out cautious about these services, and you might want to get some legal advice before using. What if you use this service, have a problem with the client and the contract turns out not to be valid in your jurisdiction?
What kind of guarantee does the website offer with respect to the use of its product?
In general, I agree with Garyrae. It's so easy to fax or email a contract these days, why would you need to use this service?