Top reasons you should not buy or use service from Vonage or Skype.

Using Vonage or Skype as your only phone line can actually be dangerous unless you have an extremely stable Internet connection. That is because you cannot call for help (such as 911) in an emergency when your Internet connection is down. Using a cell phone for 911 calls usually results in your being connected to the Highway Patrol which is a delay you may find critical.
Your phone often will not work if your Internet connection is unstable. Each time your Internet connection goes down, your phone goes down with it. This is extremely annoying when the phone cuts off in the middle of a conversation. The quality of the sound can also be very poor if your Internet connection is fluctuating or slow. Sometimes the quality can be so bad that you can't tell what the other person is saying. Remember, your broadband connection is shared with computers, servers, network printers and other network devices.
If your Internet service goes down, your phone still may not work when the connection comes back online. You may have to reset your phone router. Sometimes that doesn't work either, you end up having to reset your Internet device as well, which can be really annoying.
Both Vonage and Skype have poor customer service. You won't often need them, but in the rare cases when the service itself is down, or you are having other problems with the service, it can take a long time to contact Vonage, and they are not always helpful. Some of their customer service reps do not speak English well, and it can be extremely difficult to understand what they are saying, or to even get them to understand what you are trying to say.
Some other services don't work with Vonage or Skype. For example, the Tivo dial-up service won't work with it, and many fax machines do not work properly with it, even when you hook them up to dedicated Vonage fax lines. This is not something they warn you about before you sign up, so if you are not prepared, you may be in for a great deal of frustration.
When you first sign up for Vonage, your service may be down for several weeks until you are able to get a wireless USB connection, and wireless router. I also had many problems with my standard HP fax machine after switching to Vonage, and had to be walked through an extremely long and confusing configuration process which seems to reverse itself every few days.
They both have horrible web sites. Sometimes you have to log in on the Vonage website to manage various services or get a question answered. Their descriptions for where to go for certain information is often wrong or confusing, and many of the links or not intuitive. You may find yourself being led in circles. Since Skype is free, their service is almost nonexistant.
The FAQ on their site is laughable. When you look up a specific problem you are having with a service, you will most likely be presented with a single sentence describing what the service is, rather than how to use the service, or any common problems with the service.
You may not be able to keep your old phone number with Vonage. Even if you can, be prepared to have a new phone number for several months. Switching your old phone number to Vonage may take many months.
You have to connect all of your phone bases to a single router approved by Vonage. This does not give you much flexibility with the physical layout or configuration of your equipment. This may not matter to most users, but it may matter quite a bit to someone running a home-based business.
You can't receive calls from land lines, traditional VOIP services or cell phones. Unless every single person that you talk to and every single person that you can ever conceive of talking to and every single person who you can ever conceive of wanting to call you also uses Skype, don't get rid of your land line or cell phones yet. Why? Plain and simple, you can't receive incoming calls from a landline or a non Skype user.
Ever tried calling someone from Skype? You don't have a phone number identifiable by caller ID.
Skype is not like a Soft Phone is NO INCOMING CALLS from LAND LINES or CELL PHONES. Let’s be clear, this includes all VOIP service providers such as Vonage or Packet8.
You can't use your land lines or cordless phones with Skype. Skype does not offer the ability to bridge your wired or cordless phones to their services. So, the only place that you can make calls is from your PC, or PDA, if you have one with wireless access all over the inside and outside of your house.