While this is an interesting technology, I would not recommend that it be used for any private communications. Peer-to-peer services allow telephone calls to be routed through the privately owned equipment of one or more unknown individuals. This raises a number of confidentiality, integrity and availability concerns, and little information is available about what, if any, security controls these services have put in place to protect your telephone calls.
Would you be upset if an unknown third party was able to eavesdrop on your call? What if they were able to reroute it to a different destination? Or if they were able to disrupt your service? If the answer to all three of these questions is "no," then by all means give peer-to-peer telephone a shot. Otherwise, until the security implications are addressed, you probably want to think twice about adopting this emerging technology.
For more information on peer-to-peer VoIP security, read Skype: Its dangers and how to protect against them elsewhere on this site.
This was first published in January 2009