To elaborate on my above comment, here is how we use the Reply tracking.
We have a Contact based workflow - if Marketing email activity - replied to: “Email Name” > Set contact property: “Replied” = yes.
Next: Deal based workflow - Deal stage = xyz (and whatever conditions) AND Contact property “Replied” = yes > Set deal property: Stage = in touch.
We have workflows which follow up with people who have deals with Stage = Invited. So when they reply, their deal gets changed to “In touch”, they no longer get follow up emails.
Hope that helps make it a bit clearer for anyone who wanted to know.