WhatsApp Messenger is an iOS (and other platform) messaging app that allows notifications to be pushed instantly. It doesn't require usernames or logins, using just your phone number as your unique ID. And, unlike some messenger apps, WhatsApp Messenger is always on. The download is fast and the install is easy.
WhatsApp Messenger has an pretty standard interface with a list of clients and a chat window for each ongoing chat session. A neat feature is setting up group chats and being able to share video and music with others at the same time. Since WhatsApp Messenger uses your phone number and can read your device's contacts, there's no need to add anyone. If they are in your phone book, you can use WhatsApp Messenger with them. Since it doesn't use SMS, there's no expensive text charges to international friends, either.
We've used WhatsApp Messenger for a few months now and it has quickly become a standard messenger app. The ease of setup, the lack of logins, and having no need for server accounts make it as simple to set up as possible. Especially if you're communicating internationally, you can save a considerable amount of money on text and video chats. WhatsApp Messenger is a great app.