I am learning using the book "APL with a Mathematical Accent" and so far all the examples work fine (I am on chapter 3). I'll probably switch to learning J if I start running into problems. It says on the GNU page "implementation of ISO standard 13751" so if it's not standards compliant it should likely be filed as a bug.

I did request a copy of Dyalog but they rejected my application (probably because I didn't add my address?). I don't have a lot of enthusiasm for learning using a proprietary implementation of a language anyway.