To me it seems like it might be a kind of "silver bullet" for programming, due to a number of factors: simplicity and elegance, suitability for partial evaluation and super-compilation, and for parallelization.
I haven't tried to implement a large system in it yet though. If I get the time I hope to re-implement the Oberon OS using Joy.
I spent some time with that paper awhile back, implementing hylo-, ana-, and cata-morphisms in Python and Joy to be sure I was following along.
I loved it, but I was ultimately convinced that Category Theory et. al. isn't quite as useful for the working programmer as it should be.
What I did find useful is the treatment of recursion in the Joy language. http://www.kevinalbrecht.com/code/joy-mirror/j05cmp.html