This looks really beautiful. The fact that it's open source adds to that beauty. I'm not even interested in physically based rendering and it makes me want to play with it.
If you want to learn about state of the art rendering, check out pbrt.org. Even without the book, the source code contains a wealth of information and was written for teaching.
On the download page there is a bunch of demo scenes that you can simply open and render to see all of the gears and knobs turning and watch your CPUs light up.
Thank you! As generic_user said, feel free to just download a release for your platform and a few test scenes, start appleseed.studio, load a scene and hit F6 to start interactive rendering. The Getting Started page contains detailed instructions: http://appleseedhq.net/docs/tutorials/gettingstarted.html