I have Comcast cable and Vonage. I have to say, I'm a little disappointed. Almost every call, I get "one way audio" - I can hear them, they cant hear me. Happens about 2 minutes into the call, lasts for about 30 seconds (which is an eternity for those who aren't used to "my phone") then comes back. There's no static, pops, nothing. They just hear silence.
I've paid the $10 a month extra "speed boost package" from Comcast. I'm supposed to be getting 6 Mbps down and 768 kbps up. I have QoS set up on the firewall to give VoIP priority over everything, AND I even changed the "bandwidth saver" setting over on my Vonage account to 30 kbps (instead of 50 or 90). Still no change.
It would be nice as a second phone. Unfortunately, I don't have any other phones. They also took a long time to port my number over, I think it took like 8 weeks.