I, personally, would rather avoid using unless. It's a bit of a mind-twist as I grew up with C. Essentially it is

  if ( ! condition )
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and, for me, it is often simpler to explicitly present the not.

In fact the very posting of this question obviates the fact that unless isn't straightforward to understand for some users.

Although I accept there are rare occasions when unless might make sense it is a keyword that is best avoided when aiming for maximal maintainability.