The sheer amount of data collected and categorized from our servers and applications is amazing. The ability to see the level of detail on threads, headers, stack traces, etc. is very powerful. Add to that, the ability to setup monitors that can queue requests when resources are strained or to live debug an issue on a production server, and you have a very useful tool set.
The cloud options add a higher view level for groups and multiple servers at once. Additional alerting and metrics options are available here as well. We have only started to explore these options, but they look to add value to the overall product.
The Longest and Slow Requests feature is very helpful to our team. We can drill into the details of the request and all of the transaction elements even down to the JDBC request elements.
We initially purchased FusionReactor to help us track down a stubborn heap space issue that was very difficult to recreate in the development environment. FR played a huge role in helping us find the offending code and process. We have since renewed two years in a row, and continue to consider it a valuable tool. Review collected by and hosted on G2.com.