I find both APL and Forth fascinating. I would not try to promote them as a replacement for newer more approachable languages, but I think that learning them gives you different points of view and are worth learning for every programmer (same with FP, for example).
I still think Forth beats C as a language for microcontrollers programming. I didn't try it myself, yet, but I think a built-in REPL, nearly 1-1 correspondence with ASM and programmable compilation (among other things) can sum up to a really nice programming environment.
