I am not sure how many people are using these in a K-12 environment but at the university level thay are being used to good effect. They allow teachers to see (more or less) how they are doing, as they teach, by giving students the opportunity to answer questions throughout the lecture. The ability to quickly survey knowledge can help determine (on the fly) how the content needs to be adjusted.
My experience, having been on both ends of the SRS is that, as a student with a clicker well posed questions you feel and are more engaged. As a teacher they let you know how you are doing, which is very reassuring.
Clickers used with a interactive whiteboard, the web and streaming audio/video are a powerful (and fun) way to communicate knowledge.