My parents have had Vonage for years. Here are a few observations I've had:
1) You can't talk on the phone while downloading large files (e.g., torrents)
2) Make sure you have a good broadband connection (especially upload speed) or people won't be able to hear you
3) It used to save you a lot more money than it does now. ISPs raised the cost of internet a little while after Vonage came out, so utility bills ended up evening out pre-VoIP to post-VoIP, with people still stuck with a traditional phone now being raped. You may want to see if your ISP offers a VoIP phone bundle with your internet, but I'm assuming your ISP doesn't offer VoIP since you're asking about Vonage. (side note: my parents have had better luck with ISP VoIP than Vonage - however that may be just be a result of overall better technology over the years).
4) You are sometimes required to use a special router/modem for VoIP. These routers often suck. My folks have to power cycle their router/modem 3-4 times a week.
5) Like baja said, unlimited long distance (or even the world plan) is convenient if your wife likes to talk on the phone to long-distance friends/relatives.