For quite some time I have been using whatsapp. It is quite useful and efficient. Now I have added it to my simless Samsung Tablet also with other mobile number and enjoying the sharing and connections.
I use WhatsApp. It's not fancy, by any means-- and I don't think I'd call it the death of sms. I don't even know why it would be called that; it does the same thing. I like it because it was a free download, and I can text message my friends who are overseas. Rather than having to have skype logged in on my phone, sucking my battery and having people call me-- I can just use WhatsApp and people can text me, but not expect an immediate response (like in Skype, etc).
I've been using whatsapp for about 3 years now. Its nice because you can set notifications to your contacts so you know who's texting you. Transfer anything anytime over wifi. No need to use bandwith. The only thing that's missing is facetime of sorts.
Whatsapp is not as free as you think. My free use time is almost up and they will want a small yearly fee. Its still worth every penny.