Knock Sensor

A guy was telling me about an MSD knock sensor for my small block. Since i'll be running 11-11.5cr on pump gas, would it be a wise investment?
How do they install?
Are they actually effective, or a waste of money?
Does anybody here have any experience with them?

Here is a link to one on Summit
http://www.summitracing.com/parts/MSD-8964/