When Extreme Programming (XP) was first introduced to the VCAPS project there were no automated acceptance tests. It took a while to add coverage. We added tests for all new functionality and old functionality as it required changes. After about a year we had an estimated 40% covered by tests and the trouble ticket count dropped by 50% as well. We don't consider this a coincidence.
But the customers had noted a different effect. By trapping bugs before they reached the production environment there were far fewer emergency production releases. Previously, it was not uncommon to release to production a couple times a day for a couple days due to bugs that required an immediate fix. The acceptance tests improved the quality of the system to a point where a production release was rarely re-released because an emergency fix was required.
The customers experienced this as a system with a far greater feeling of stability. They had more confidence in the system and us. The customers also noticed that with fewer releases there was a large drop in spurious bugs often caused by quick and dirty fixes. Next Lesson