We used AppDynamics originally for application performance monitoring of our custom websites and applications. We didn't know where the slow-downs were happening. The up/down way of monitoring wasn't enough for us, so the slowness, machine learning and baselining was really nice. Seeing successful, yet slow transactions really helped us find problems. It was nice to see the "inside-out" view of our apps.
This was an added service, so though it wasn't fully deployed, we used it across various departments for a variety of applications.
Seeing user experience was also very helpful. We purchased the end-user monitoring portion of it to capture "outside-in" and marry that with the previous view from the APM portion.