iMessage here. Only need it on Windows now to complete the cycle.
I, for the 1 billionth time, still don't get the appeal of WhatsApp. Tried it before it was released and after. It requires everyone you know to have it. Everyone I know, doesn't have it. They all have iPhones as well. What's the point? I guess I can understand it's use for non iOS users?
You can e-mail an entire conversation thread, in case you want to save a conversation or send it to someone else, or just back it up.
You can look at all the media sent to the conversation without having to scroll up and load it first.
You can actually quit a group conversation.
You can turn on/off notifications for individual conversations.
I mean, WhatsApp is the Batmobile and iMessage is a moped. If all you need to do is send a message or two from time to time, iMessage is definitely enough, but not understanding why someone with iOS might like WhatsApp is just being plain clueless.