The biggest reason I balk at CEGUI is its apparent reliance on its built-in renderers and resource loaders.

CEGUI gives you a set of renderers for common graphics engines, but there are few resources on how to write your own renderer like there is for, say, libRocket. And I'm hesitant to give CEGUI control over its text rendering when I already have text rendering in my game (libRocket falls a bit on this too, actually). How feasible is it to create a custom renderer, especially since I apparently only have the CEGUI::Renderer class reference to go on?

I'm also worried about CEGUI's data files and resource loading clashing with my own. CEGUI wants Imageset files, while I already have a sprite atlas system in place. CEGUI uses XML files (which requires an XML library) while all of my existing data files are YAML or scripts. How much of CEGUI's native resource loading is required for CEGUI, both for compiling and using CEGUI?