F**king Lisa man... I've never heard my sister shriek that loud. I really think first person is essential. It just ups the terror factor, if you're basically in the shoes of the protagonist, and not just a 3rd party, looking over his shoulder.
And some things just work better, like the words constantly changing, when you weren't looking. That wouldn't have worked as well in 3rd person. You'd have to turn the entire character around in order to get the camera angle away from the wall, and then turn the whole character around again, back to the wall. And there's a part where you have to do it several times, because each time you look away, a letter disappears.