From the Google style guide:

"The semantics of this can be tricky. At times it refers to the global object (in most places), the scope of the caller (in eval), a node in the DOM tree (when attached using an event handler HTML attribute), a newly created object (in a constructor), or some other object (if function was call()ed or apply()ed)."

The pragmatic answer is to limit its use except when absolutely required (as Google's guide suggests). The more problematic meta-problem, is that many people do think it's valuable and don't understand it and dig themselves into some subtle and insidious messes.