I really dived into all the options and features of this tool, but it helped us understand that we're burning money in the wrong places (made us leave App-Engine Flex in favor of GKE). Specifically with App-Engine, I remember that the tool didn't show us which service costs more than the other, but maybe there were some configurations that we missed.
Overall it's a great way to track our GCP spending. We use labels to mark resources for various sub-projects and it's great we can track per-subproject spending this way.
The setup is basically 1-click. It gives very useful insight with clear presentation.
Actually, google should certainly buy it and offers it.
I use it for analyzing GCP costs in general, per project, and specifically for BigQuery usage analysis per user and per query. It helps me find heavy queries, users whose queries can be optimized and in general optimize performance and costs.
We are using reOptimize for the last few month and Its a life saver