I've been with Vonage's $25 Unlimited (local, long distance, voicemail, call waiting, etc...) plan for the last 6 months or so. No problems at all. I was paying $83+ for the same service with BellSouth, who keeps sending me offers to come back for a mere $60 or so. No thanks. Everybody in the family has cell phones with excellent coverage, so a power outage isn't much of a concern. As a matter of fact, with Vonage we set up the account online to have all of our home calls forwarded to my wife's cell phone in the case of a service interruption, so we never miss a call, even if the power, cable, or Vonage service is out. We were also able to keep our old phone number.
Just remember that this is a fairly new technology, so there are inevitably going to be some snags along the way, and most digital phone companies are growing faster than their customer service departments can handle.