The only people who refer to it as parens hell seem to be non-lispers. As I understand it m-exps were never implemented because people don't really want them. I definitely prefer s-exps over them.
It seems plausible that the implied causality is backwards - perhaps it's not that people don't mind the parens because they use LISP, but rather that people use LISP because they don't mind the parens. Personally, I don't think I ever did mind them, but I'd be really surprised if that's not at least a moderately important filter on the set of people who become lispers.
