We just signed up for Vonage's consumer voice over ip service, and I thought I'd give it a quick review
$39.99 a month, flat fee, unlimited usage anywhere in the US and Canada. That includes call-waiting, voice mail, caller id, etc. They have a $25 plan as well, which has some limitations on the number of minutes you can use.
Configuration was simply plug and play. The only problem that I had was that I left the ringer off on the phone that I initially used -- so I mistakenly thought that I was not receiving incoming calls.
You need broadband internet access. A minimum of 90 kbps up and down is required.
Although they can let you keep your existing phone number in some cities, we couldn't keep ours. In fact, although we were able to keep our area code, we couldn't even keep our local exchange. (Their customer support folks claim that this will change in the next few months.)
You have to dial all 10 digits to dial any number (even local calls).
They don't support 911.
I occassionally notice a slight echo of my own voice, and (occassionally) you can't hear sounds from the other end of the line while you are speaking. (I have 1.4 mbps down / 300 kbps up.)
I have to buy a new wireless expansion phone, because my LynkSys switch is on the 3rd floor, and we want a phone on the 1st floor.
That's a substantial number of drawbacks, but it's still worth it to me, considering that our phone bill was $170 last month.