I've had Vonage for more than a year now and so far so good. We had a problem with our router awhile back but the tech support people were very good in working with us to resolve the issue.
The clarity of the call is so much better than what my landline used to be. We did have an issue with the calls with echoing for a little bit at one point, but it turned out to be a conflict with my torrent software. That's also been resolved.
At this point, I'm really happy with them. I love that I can check voicemail from any computer with internet connection, even when I'm in WDW!