Telephone vs VoIP

As a one income family, we're always looking at possible ways to save money.  So this story (free reg required) in the Washington Post intrigued me.  VoIP is becoming increasingly popular for business and individual use, with some people completely dropping their standard telephone lines in favor of using internet-based voice communication services.

I'm curious....  Would VoIP be cheaper?  How's the sound quality?  Anybody out there have some advice on telephone versus VoIP?