It's pretty damned effective. The company I work for has offices in 32 countries and uses VoIP exclusively from all of them. The quality and security has come a very long way and remember that standard POTS has zero security.
For my own personal use I have skype (
www.skype.com) which allows you to talk for free to anyone in the world that also has skype and to make calls to standard telephone lines for nominal per minute fees. I use to talk to the States all the time from Guatemala at US2.9 cents a minute. The quality is exceptional even on a very poor connection.
Hope this helps.