I have the unlimited Vonage plan (just reduced to $24.95) and it works pretty good. I haven't had any outages. You get all the extras that the phone company likes to charge for like voice mail,3-way calling, caller ID, etc.
There are a couple of cool things as well. You can have your calls to your vonage number simultaneously ring your vonage phone and another phone.
You can have your voice mails emailed to you and play them on your PC or you can access them from the website.
You can have almost any number you wish. You can get a second number for about 5 bucks more and that can be almost any number as well. If you have relatives or friends in another state, you can get a number that is local to them. If you get the unlimited plan from vonage, all calls to and from your friends or relatives are at no extra cost to them or you.
You can take your adapter (you need a phone too) with you and hook it to any broadband connection and receive YOUR calls. The phone number is assigned to the phone adapter.
One thing you need to have is a broadband connection. You will also want to connect the phone adapter as close as possible to the cable or dsl modem. It can be behind a router but you don't want it to go through multiple connections if possible.
The downside is that you don't get direct 9-11 calling. You need to "register" yourself and provide your information as the call center won't know where you are automatically because 9-11 calls don't go directly to the 9-11 call center. It goes to the emergency services number for your area.
Hopefully that will change as the service gets more popular.
One other downside that I've seen and it may be the way I have it set up is that I sometimes get a little glitch in the audio if I'm doing heavy internet downloading. Doesn't happen all the time but it does. Having said that, I find the service much better and the quality consistently better than cell phones.