I have license and I have contact to users using this and other engines like Unity3d and asked them as well.

C4 is superior when it comes to lighting and rendering features. Unity3d as an example is more easy in terms of editors and workflow.
C4 offers a visual node-based scripting editor, level, material and model editor. They are all powerful and provide fantastic lighting (several types of lights and shadows and all are fast).
The scene management with portals allows to create big levels, much better for indoors than Unity3d or Gamestudio while providing modern visuals at the same time.
The panel/GUI system is told to be very easy and powerful. There are interactive panels in the demo and a panel editor.

But the community is smaller but they help if you have questions. You can even talk to the programmer (Eric).
If you want to add more functionality then you have to know C++ with good understanding of it's object orientation.

The announced new features of the next edition are amazing, including streaming for even bigger worlds without level-load, more shadow-options, more post-processing including HDR and so on.
In terms of technology this is the best indie engine out there.