Why I like WriteRoom
I do much or all of my "serious" (aka "paid") professional prose writing in Scrivener. It lets me collocate my writings, offers a great writing mode, and generally stays out of my way and Just Works. Still, I've spent many a lengthy night starting at the BLANK SCREEN OF DEATH any writer's faced when they're just struggling to begin that journey of fifteen-to-twenty-two-hundred words with that first word put down. When I'm on one of my OSX machines, Scrivener's full-screen mode clears away distractions and makes it easier to get off the schneid. Before I found Scrivener, WriteRoom did this for me, and version one was free-as-in-beer, I think... I never paid for it, in any case (though I gladly paid for a license for Scrivener).
Sometimes (like, say, for example, from 8:30-5pm on days between Monday and Friday, just to pick a few :), though, I'm stuck on a windows machine with no Scrivener access. Fortunately, there's Dark Room, which is basically WriteRoom for windows, but even better, because it's (IIRC) GPL'd.
So, to answer your question, it's saved my tuchus on numerous occasions, and it led me to other apps with similar features which continue to save my tuchus, so I gladly recommend 'em all.