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.
Thanks to Arnulfo Hernandez Pulido one of our Front End and Apigee Developers and the rest of the team at the MDC for diving deep into behat documentation and being able to write custom context and test plans. Each team member can now write and run behat test locally and remotely in pantheon as an anonymous users
What is Behat?
Behat is a tool that makes behavior driven development (BDD) possible. With BDD, you write human-readable stories that describe the behavior of your application. You can follow the instructions at http://behat-drupal-extension.readthedocs.org/en/3.1/localinstall.html to install and give it a test run. You can also use https://www.drupal.org/project/drupalextension for a quick start using Drupal