no quality issues whatsoever as long as i set the bandwidth usage to a low level. at higher bandwidth, it seems the vonage adapter uses too much and the TWC modem can't provide enough.
on another note, extra fees and taxes have really soured me on the great "deal" that vonage offers. i pay with the once per year option to save extra money and i think i still pay over $30 a month. great features. great quality. the cost, however, i think is going to drive me to cancel after 4 years of service.