I loved Vonage in the States. It was also good here up to about 2 or 3 months after I moved (we had it for about a year before I moved here so that we could both call for free).
I don't know if our current problem is because of the setup here (my brother-in-law has it in his room) or if Vonage quality has gone downhill. Both my sister and my parents in the States have had problems with it. The problems my parents have could be blamed on technical inexperience. My sisters' problems, not so much.
However, when I was still in the US, Vonage was great with fixing problems that came up. Again, not sure if this is still the case.
Advice:
1)Read the contract.
2)Be on the lookout for auto-renewals. If you are planning to quit the contract, make sure you know when you can.
3)Make sure that the people who are using it know how to set it up, reboot routers, check for problems, especially if you are not going to be around to help them out.
4)Make sure you know what's covered and what's not. Mobile numbers, premium numbers, etc., all matter. Also, not all plans cover all countries.
5)Make sure your internet connection is up for it. I had a standard cable connection, and I guess there were some sort of filters slowing down my connection. Time Warner had to come out and fix it (Adelphia originally set up the connection).
6)If you are replacing your landline and you use DSL, you will still need a phone line for the DSL.