The service is straight forward and easy to use. You basically start with a picture that you upload (or one that is already on Facebook) or choose from a selection of post cards. You can then write a personalized message, followed by the recipients address. You can even choose which advertisement will be printed on the back of the card. One area that could be improved would be to automate the filling out of the address by letting you choose from your friends on Facebook that have their addresses visible.
Update: This story is being covered over on Techcrunch per my suggestion! I guess the privacy issue isn't a big of deal as I made it out to be because a member of the Hippopost team stated on their forum that they do not sell or give your information to their advertisers.