My first reaction to Gogol was bewilderment. It's funny, and engaging to read, but...what the hell is it about? I'm not sure what the point of "Diary of a Madman" is, although I know I enjoyed it.
Pevear and Volokhonsky's intro is helpful, although it contains a number of minor spoilers. Their point is that if you try to understand Gogol, you are failing: Gogol himself didn't understand Gogol. "We still do not know what Gogol is," says some guy they quoted. P&V write that Gogol, as compared to traditional storytellers, "has nothing in mind. Memory plays no part in his work. He does not know where the act of writing will lead him."
Pushkin, an early and ardent supporter, wrote, "Here is real gaiety - honest, unconstrained, without mincing, without primness. And in places, what poetry! What sensitivity! All this is so unusual in our present-day literature that I still haven't recovered." And that seems fair to me. It's still unusual now (although at least we have Borges); maybe we should shut up about what it means and just have a good time with it.