The UI, ease of use for my team to be able understand what's going with our applications in real time! It has been great so far as we have zero issues or failures when using it across multiple teams at different locations all over US & Europe.
There are some things I think could improve like adding more dashboards per application (like MySQL). We're solving problems around availability/reliability by having visibility into how many requests each app receives from users every minute - this allows us to make adjustments if needed without affecting end-users' experience too much.