It's been pretty much a disaster. The quality of the service when it's up isn't bad. The price isn't bad. But the reliability is awful. In six weeks we've had five outages. The first time I called Vonage tech support and spent better than 90 minutes being talked through the rote "reboot this restart that plug this in" script. In the end, after finally getting transferred to second level support, I was told that it was a provisioning problem and would not happen again. Only it has happened again and again and again.
Seems that Vonage is all about attracting new customers (witness their TV ad campaign, which has got to cost pretty large $$) and not about improving or maintaining their infrastructure. There's no reason I can't get decent uptime with this solution. They're a freaking phone company, I expect 5 9's reliability. They don't come close to that.
Now it's a choice between abandoning the landline altogether and going pure cell or finding a different VoIP provider. Reading VoIP reviews is not very helpful, as it seems that every provider has fans and haters and it's really tough to tell who's going to be the most reliable.