At my current project, we have been experimenting with Behat. We are currently using behat to test key user stories before and after every code push in pantheon. I will be posting more information later on the entire process and how we got it to work well for us. In the mean time, I have attached a screen shot from a sample test to give you an idea of what it may look like for you and clients when you do it right. We have basically turned client user stories into functional test plans that produces results that are clean and easy to understand by the client.