My company will release their VoIP offering late Q4 of this year. I've spent the better part of 3 months visiting the manufacturers and designing our architecture. These phones have some uncontrollable varialbes that can affect QoS. The biggest one is the quality of your broadband connection; what your throughput is, your flutter, and how many hops between you and the trunk gateway they connect you to (they use trunk gateways instead of true next gen phone switches). Depending on the Codec standard Vonage has you running on, your audio quality should meet that of a cell phone with a good signal.
Some things that differ between a VoIP phone and a land line phone:
e911: since this a phone that has it's ;ast mile routed through the internet cloud, there is no way to trace a call to a location. You can register an address with your provider, but read the T's and C's. Vonage makes NO committment for 911 response.
CALEA: this may be a bonus to some of you. There is no way for a traditional phone tap to be set up on a VoIP call. the audio is sent via IP packets, there is no actual call to listen in on.
Directory assistance: always third party if available. Not a deal buster, but should be aware of this deviation from the norm.
I've tried to water this down to a point it is understandable and hits the issues most consumers are concerned about. If you want any more info, feel free to email me. I'd be happy to answer your questions.
Edited to add: don't rely on Vonage for your business, we'll be bundling a dedicated T1 for voice and data utilizing our VoIP services. No issues with the last mile stuff.