Strengths: there are many strength when using Voice Over Internet Protocol. For instance I suspect for most people who have relatives abroad it is a good easy and its usually free way to stay in contact with them without running up a huge and unnecessary phone bill. Voice Over Internet Protocol also allows you to speak to your friend or relative for as long as you wish without charging you anything at all.
Weaknesses: as well as having many strengths do come a few weaknesses as well. As you are communicating over the Internet when using a Voice Over Internet Protocol, there could be some interruption because of bad connection, and if there is too much interruption with your connections you may loose your connection or may not even be able to connect at all.
Practicalities: Voice Over Internet Protocol is a very practical way to stay in touch with friend, family or even a business associate, as long as you both good connections with each other its just like being on a phone just usually with no cost depending on which site or format of VoIP you use.
Usefulness: Voice Over Internet Protocol is a very useful to communicate with other people, for example if your friends phone is turned off or engaged you can send messages through your VoIP site or programme you use to speak to people online to let them no you want to talk to them. Obviously its not as useful as a mobile phone as a use of communication whilst on the move or out, although Voice Over Internet Protocol Is good whilst at home so you could never really see it converting from normal calls to VoIP at least not for now anyway.
Public and business usefulness: Voice Over Internet Protocol is very well used now all over the world more than you might think. It may not be used extremely well through online applications such as Skype, but it is used astronomically over online gaming and Xbox 360 have made a very successful business through this. Xbox charge £39.99 for Xbox live for one year, which primarily allows you to play and speak to other people online and even speaking online in groups of people not just one to one. Billions of people have used and still use Xbox live at this £39.99 charge, which has created a very viable business for Microsoft.
You can also see Businesses using Voice Over Internet Protocol when speaking to business associates abroad or even just to discuss business deals through programmes like Skype to save on the price on calls, which can go back into the business.