I never had problem with the service for what it was, but I did have issues with the hardware.
The router they sent me does not play well with other VoIP providers, or at all. I use a business oriented VoIP service for work, and it refused to work at all plugged into the vonage provided router, the only way it would is if the vonage router and the other voip phone into my own router seperately, which i thought was a pain in the ass.
Aside from that I liked the service, but shame that all the telcos and other such companies are gutting it in court.