Yes, APL was an amazing language. It was also way ahead of it's time. The technology wasn't quite ready for it back then. One can only wonder what might have happened had it been introduced thirty years later. The language simply didn't have an opportunity to evolve and mutate into something that could be pretty amazing by today's standards. If you've never used APL none of this will make sense to you.
And, no, J and other variants are equivalent. They really didn't contribute to the original concept of the language. As I said in a prior thread, given what Iverson says in this paper it is hard to imagine J as anything other than a lame attempt to reach some degree of financial success in an era when doing a language that was based on the concept of creating a notation system for computing was just about impossible. J, in many ways, is an abomination that may have hastened the lack of interest in APL.
Not sure a renaissance is possible today despite seeing posts on HN every few months.