Copper was just released, but it's already very useful for assembly of boards and visualization (my use cases - I haven't played with the other features yet).
The visualizations alone make it much easier to work with boards:
- I can find all components with the same package/value quickly and highlight them for assembly with just a few clicks (I've been looking for ways to do this in EAGLE for a long time);
- I can find signals quickly, which is really useful when debugging circuits ("what's _this_ trace again?" or "where can I find signal X?");
- I can view the board in 3D and export a fully populated 3D model to other CAD applications to design enclosures/systems around them.
Also, the support staff has been very responsive and helpful so far, answering questions/feature requests and adding missing 3D models to the library.
4/5 stars because the app could use a bit more polish/performance improvements, but nothing major really. It's a great app as-is and will be a solid addition to my workflow.