GNU APL is unusually polished considering how obscure it is. When I was first looking for a free APL implementation some years ago, it took me quite a while to find it via search engines. It also doesn't seem like it's very visible in the APL community itself.
In fact, my only criticism is that it is a bit awkward to write simple Unix-style programs. The APL interpreter quite resisted being used in batch mode, and needed special flags to avoid launching background daemons or being chatty on the standard output streams. That was a few years ago, though - I wonder if it's been fixed.
Edit: looks like the search engines are much more favourable to GNU APL nowadays.
I agree with the difficulty in writing standalone programs. However, this is not the main use case for APL. You should think of it more as an alternative to something like R, which also isn't normally used to develop arbitrary standalone programs.
I use GNU APL as a smarter calculator, allowing to me manipulate datasets in a very flexible way. The programs I write a small utilities that I user interactively.
That said, the FILE_IO workspace provides access to low-level OS access. It can be a bit cumbersome to use, but it's good for building higher-level abstractions. As an example, last year I made an APL submission to Google Code Jam and while the solution to the problem was a single line of code, reading the data file and writing the output was significantly more.
In fact, my only criticism is that it is a bit awkward to write simple Unix-style programs. The APL interpreter quite resisted being used in batch mode, and needed special flags to avoid launching background daemons or being chatty on the standard output streams. That was a few years ago, though - I wonder if it's been fixed.
Edit: looks like the search engines are much more favourable to GNU APL nowadays.