We have been using Vonage as our second line (fax line) for years and it's as reliable as your cable or DSL service is. Still the most reliable is the traditional land line and we'll never give that up.
I have had Vonage for about 2 years now, everything works great with it, like some others have stated here already, after you have your phone working you need to register it for the 911 system to store your address. I have never had one problem with it and we love it. The only time it has been down in the two years is when my fiber internet has been doing an upgrade and that is very rare. Good luck with your choice but Vonage does work great.