Kik gives you three levels of status for a message - confirmation that it's been sent, confirmation it's been delivered, and confirmation that it's been read. That's better than SMS. You can also add photos without worrying about MMS charges, which is nice. (MMSes are not included in a lot of plans, especially here in the UK)
I suspect that if you have unlimited texts, the main advantage is not directly for you, but rather for those people you contact regularly who do NOT have large amounts of texts in their plans.
the problem, in my oppinion, is getting your friends to start using the app. It wont do you any good if you're the only one using it. None of my freinds seem to use WhatsApp or Kik, or anything like it.. so it's of no use to me