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.