I have had Vonage for over 4 years now and have had no issues with it. I have it plugged into my router so I can power down the computer when I am not using it. My father got it after I had recommended it to him and he has connectivity issurs with his brother-in-law at times (they both live in the boonies of PA), but it works fine other than that, so I don't see that as an issue.
I have cell phones for when the power is out, and car chargers for them if battery is low. Only problem with the cells, would be from large scale black out where the cell towers are down.