I have a couple coworkers and a friend that are both Vonage subscribers. They both like the service.
I've balked at it, because if my internet goes down, I have no phone. I want a landline to use for 'net access (since it's used for work) as a backup to dial up to the 'net.
A client subscribed to Vonage for phone service in their store. Their 'net connection was DSL through the phone company, SBC. They really liked it. In fact, they liked it so much, they called SBC and had all their phone lines disconnected, including the one for their DSL connection!