I've looked at it before, though not recently. Honestly, a big part of my library is just for becoming fluent in the quirks and limitations of Coq. I like to have a project to work on in order to learn a language. I don't have any good problems to itch at the moment, so category theory seemed as good a project as any. In that regard it's rather different than the Vecs library which is a practical library to fill an apparently vacant niche.