Leiningen gets messy when trying to define complicated build paths combining Clojure, ClojureScript and possibly multiple artefacts, CSS compilations etc in one build process/configuration file.
Since Clojure 1.7.0 there can be support for multiple platforms in one codebase through cljc-files. This is awesome but must be leveraged.
Boot solves a lot of these problems by composing builds just like ordinary functions. To overcome problems with mutable file systems, boot uses an immutable file system called Filesets. Very clever.
I'm investigating wheter a semi-complex build process would benefit from moving from two repos containing a backend server and an administration client respectively.
I found out that there's no selective auto-reloading of code like in the amazing Figwheel. This limits the usability of boot for doing rich web app development at the moment. I'm sure this will change very soon.
I'll stick with Leiningen for some more, but keeps my eyes wide open for improvements in the Boot project.