Answer:
Reliability is quite poor. Syllables and entire words will frequently drop out on both ends of the call. Conducting a telephone conversation through Vonage connection is very difficult, as a result, as both parties are constantly asking the other party to repeat there last sentence.
The "Vonage Device" will frequently lock-up and results in a loss of phone service. Power cycling restores service.
Customer service and technical support are horrible. It starts in India and when you threaten to cancel service, you end up in the USA. However, the knowledge of their USA support staff is lacking, as well.
Vonage's "Terms of Service" is a one sided document written for Vonage. In short, they guarantee nothing and warrant nothing. When you finally have enough of the unreliable, worthless, service; you will likely have your credit card billed for at least $100 in termination fees.