I have two Vonage devices. I have one at the colo because it was cheaper to pay the $14.99 than it was to pay the $50/month crossconnect and $30/month for a landline. I didn't use that one for much more than dialout notifications.
I got the second one after Frances since my DSL was working during the hurricane, but, cell & voice were dead. During Jeanne, DSL also died
I like it, although until this month, I haven't really been using it much. Its $.04/minute to call Australia, $.06/minute to call Poland, etc. Its still $12/minute to call InMarSat phones haha. As far as a normal landline, its not too bad. There are some nice features that I don't use (email notification of voicemails, forwarding, conference which has come in handy once) and a few other things. There are periods where it goes offline -- presumably because my DSL connection chokes it and it cannot check in.
When I am downloading things, calls get a little choppy -- I just set up QOS on my linksys router for the port that it is hooked to so that it always gets priority when it is in use. I'll have to see if it makes a difference. Except for the savings on the International calls, a cell phone is roughly the same price and much more functional. They have a 50kb/sec setting, but, some people said the quality was a bit too low. Not sure why, they listen to 12k/sec mono soundtracks on porno films...
They do have a plan for unlimited US/Canada calls -- and I did take my vonage device to a hotel and jacked right in with my number. You can then plug your machine in behind it so that you can surf through it transparently -- makes it nice when you forget to bring a hub or have to log into their application before the port is turned live.
Overall, I like it. I prefer a phone to a headset contraption on my sound card. The fact that I run Linux on my laptop also makes some of those services like skype a little cumbersome at best.
Their dashboard application is pretty nice -- calls show up within a minute of the end of the call so I can see just how much time I was on the phone. If you wanted a phone, and your bandwidth at home is pretty stable, you shouldn't have any problems. The only issues I had were when I was uploading/downloading something while on the phone -- normal surfing/checking email didn't seem to make a difference.
Another feature they have is the virtual numbers. You could get a number from some other city, and it would be like a local call for them. Or I could get a Canada number and you could call a local number, and my phone would ring. I forget how many numbers you can stack on there, but, you can have NYC, LA, etc and your own number and they all ring through like a normal call to you, and the caller in those cities is making a local call.