We do all development on Cesium in feature branches, which get peer-reviewed and merged into master when they’re fully tested and ready to use. You are certainly welcome to try out branches containing “in-development” work (I know the imagery_layers branch is of interest to a lot of people), and we welcome any feedback you have, but be aware that they may be incomplete or have broken functionality.
In particular, the imagery_layers branch has terrain and layered imagery, but 2D and Columbus View have been broken while we focus on the re-architecture in 3D. There are also a number of branches containing some experimental prototype work from our recent hackathon at AGI, as well as other, more mundane branches containing smaller fixes or improvements related to open issues, etc.
So, in summary, feel free to try the other branches to see what’s being worked on in Cesium, but watch out for problems along the way.