I was just reading somewhere of a playwright who wrote a snarky email to a critic who had (in the critics' own words) "savaged" their writing.
This made me think of my recent response to a critic who off-handedly charged me with plagiarism.
I would (and this seems strange) side with the critic in the first case: a playwright should NEVER respond to a negative review, no matter how unfairly they feel they might have been treated.
The critic has the bully pulpit here, and any response will just make you seem petty, at best.
If anything, it only empowers the critic, and gives their review more weight (and attention).
I would only make two important exceptions to that rule (though there might be more that I haven't come across yet):
1. The critic accuses you, or your writing, of something wildly damaging to your career/reputation - that is completely untrue - thereby forcing you to respond. (ie, my recent missive re: Grimm. I would also include the recent "homophobia" charge against the author of Johnny Baseball as an example: it just wasn't there, was a serious charge, and justified a response.)
2. The critic is anonymous/uses a pseudonym. This is WILDLY unfair, and a clear conflict of interests. It happens more and more now that blogs are everywhere. I remember reading of a recent case covered in The Times of a woman who sued a web-site for the email of an anonymous (and slanderous) blogger/reviewer, and won. It turns out, of course, that the blogger was a fierce competitor in her field.
A reviewer MUST stand behind their words, as any writer does.
This said, it is always a tricky situation to respond to your critics.
My best advice, unless you find yourself in one of the above situations: ignore them.
And even then, you might still want to ignore them.
Or even better: write a play about them.
Tom Stoppard's excellent The Real Inspector Hound (which The Publick will be performing in the fall at the BCA) is a great example of a playwright writing about (and alternately humanizing and savaging) critics, as is Conor McPherson's St. Nicholas, a terrific one-person play the Sugan produced years ago, about a theatre critic who falls in with a troupe of actors who are really vampires.
The truth is, we NEED more people writing about theatre, who care about theatre, who are passionate about it, who want to start a conversation.
Even if we don't always like to hear what they have to say.