I have a really hard time parsing `unless`. Whenever I encounter it in Ruby or CoffeeScript code, it takes me 10-30 seconds to stop and understand what's happening with the code.

Generally, postfix conditionals are a bad idea because they are garden-path sentences [1] by design.

Glad you find them valuable, but IMO they epitomize what many other comments in this thread have said about CoffeeScript's poor readability.

[1] https://en.wikipedia.org/wiki/Garden-path_sentence

I don't see anything common in structure between postfix `unless` and examples of garden-path sentences, which seem to be based on word ambiguity. Nothing is ambiguous in e.g. `print(x) unless x != 0`. Although admittedly without syntax highlighting, conditionality it's less obvious.