Just wanted to give my (experienced) opinion for some of you that may be considering going to VoIP.
It rocks, but it has some gotchas. The technology is well developed, and it is being used extensively in some large companies due to the cost savings, and call quality that is comparable to land lines.
The things that most large businesses have that consumers won't however is dedicated bandwidth on private networks. The problem with home VoIP is that the voice packets that travel over the Internet can experience large latency (delay), which can make your call choppy. One thing that will help this is if you have a phone adapter with built in QoS or Quality of Service which will allocate more bandwidth for your voice call if it needs it. This only applies if you have people online surfing when you are using the phone (it give the telephone priority).
I have such an adapter from Vonage, and it works; most of the time. During times of high "neighborhood Internet surfing" (Comcast puts neighorhoods on one big shared network) 6-9pm for instance, I often get choppy calls, but at other times they are fine. Some of this depends on your ISP and how much bandwith you have, but you are still at the mercy of the Internet, which is very busy nowadays, and is full of latency (how long packets take to go from point A to B).
I call Australia often, as well as a other states, and it doesn't seem to matter where I call, I get great call quality (during those non-peak times).
Things to watch out for :
If you have DSL be wary if you are trying to drop your local telco; often they make you have a phone line with them in order to have their DSL service.
Make sure they have good 911 service, it functions like normal 911, and they don't charge extra. (Vonage has this, was one of the first, and it was easy to get up and running).
Make sure you are willing to put up with poor customer service and tech support. Most VoIP providers are notorius for bad service. I would say unless you are an expericenced and knowledgeable user, stay away from VoIP, as it requires a lot of patience and some troubleshooting.
If you switch phone providers to VoIP your from local carrier, if you lose power, you lose your phone. Local telephone carriers supply power to your telephone, and have backup systems so that your phone works during a power outage. If you lose power with VoIP, better have a charged cell phone.
If you lose your Internet service, which I did several times a couple of months ago when Comcast was having problems, you lose your phone.
If you have a security system which calls out, it will likely stop working if you go VoIP. There are ways to fix this if you're willing and able to go through the hassle. If you use VoIP as a supplemental line, and keep your regular phone line, you can avoid this.
Unless you can do your own phone wiring, you will only be able to use one phone which plugs into the adapter (you can always use those multiple handset cordless phones).
VoIP is a mature technology and can save you a boat load of money (I save $30 a month), but you have to be willing to go through a lot of trouble and put up with a lot of downtime and choppy calls. The Internet is now a crowded network which can really mess with your call quality during peak times.
I have several friends I wanted to refer to Vonage, but I am holding off. It still makes sense for me, and I am still happy with Vonage overall, but only because I do this for a living.
Hope this helps