This is an API, not an application. In other words, if you want the end-user to get an SMS reply, you have to write code and instruct the API to send a message.
Mobivity gives you simple marketing tools (voting, pre-defined messages to reply with, etc.).
Actually, no, Mobivity give you an API to send and receive SMS messages, and they do it in pretty much the same way this site does. I know this because I've used them to do exactly that. I know it does the the marketing as well but it has a fully fledged SMS api too.
Mobivity gives you simple marketing tools (voting, pre-defined messages to reply with, etc.).