Great for conversations among multiple users
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.
A note about pricing
WhatsApp Messenger is free to download for iOS and Android users. In the past, the app cost 99 cents, but the developers moved to a newer pricing model in the middle of 2013. Now, the app is free initially, but after a year of use, you'll need to pay a 99-cent subscription fee for another year. This is the first time I've seen such a pricing scheme, but it's hard to argue with only $1 per year for a solid messaging app that works cross-platform.
Overall, WhatsApp Messenger is not much different from the text-messaging services you get on your smartphone, offering a few neat extras that make it a little more fun and useful. At this point in time, it's hard to say what will happen now that Facebook is taking control. Until we find out, if you do a lot of texting, this app is still a must-have, giving you unlimited free texts with all your friends, whether iOS- or Android-using, without having to worry about extra charges, even internationally.