Their Gitter channel is absolutely kicking, where lots of the actual devs, including CTO and co-founder Adam Gibson, hang out and /really/ answer /any/ question - I find their responsiveness stunning! (I discussed and reported a minor improvement to the config builder, and it was implemented - literally - the very next day!)
Just recently found the library myself, and I found it very refreshing to be able to utilize my decent Java competence in ML, instead of continuously having to hammer my way through limited Python experience to get things done. (Python also have limited multi-threading capabilities which I found annoying when wanting to do some larger ETL stuff "inline". Python also gives me a strong feeling of "this is a scripting language!": It is super fast and expressive when your needs are met by the vast set of (native) libraries - but when I just want to do something myself with raw code, things gets slower. I also love a properly typed language, which python obviously isn't!)
Do notice that with dl4j, you still get full-on GPU acceleration via Nd4j, which is a "NumPy-style" multi-dim array system.