One thing WhatsApp does exceptionally well is group messages. You can use the Broadcast Message feature to bring up your entire list of contacts, then click radio buttons to quickly add contacts for a message blast. There's also a New Group feature, which lets you add contacts for a group message much as you would in iMessage or another chat app. But you can also look back over the group messages and list just the locations or just the media added to the group chat by all members. These are not amazing or groundbreaking features, but they are a bit more than what you can get with the standard messaging features on most phones.
So the real advantage of WhatsApp Messenger is the money you save when communicating with
Android( if you use iMessage) or while communicating internationally. In other words, with WhatsApp Messenger, and a little cooperation from your most-texted friends and family, you could easily get the lowest-cost texting plan with your carrier and save some money every month on your smartphone bill. It shouldn't be that hard of a sell either, since your friends will save money on their phone bills as well.
