I really enjoyed the concepts in CLIM back when I programmed in CL. These days I can't even imagine using CLOS and relying on mutable state in objects. I'd much rather have my UI be a function of data, which is why I enjoy using React (with ClojureScript and the Rum library). But there are many ideas in CLIM that are worth at least learning and thinking about (for example, Presentation Types and presentations in general).