I have been a programmer, scientist, etc, at various times in my life and I have programmed in J. I don't think there is any compelling reason to use J over Matlab, R, or Python and very many reasons not to. Vector languages are mind expanding, for sure, but they have done a very poor job keeping up with the user experience and network effects of newer languages.
A few years ago I wrote a pipeline in J and then re-implemented it R. The J code was exactingly crafted and the R code was naive, but the R code was still faster, easier to read and maintain and, frankly, easier to write. J gives a certain perverse frisson, but beyond that I don't really see the use case.
