[__jem]

Interop from Clojure -> Java is still incredibly easy, though. I often will just wrap a Java library rather than look for a pre-existing Clojure implementation because it's so easy. Of course, most Java libs don't value immutability and functional patterns, but you can still push this kind of interop to the edges of your system and keep everything else in pure Clojure.

[StreamBright]

Depends on the style the Java library you are trying to use is written in. I have had mixed results. Especially the Java 8 functional style had some challenges. I might have the relevant SO question somewhere.