I'd counter what some other people have said and tell you that you should buy it. It's not such a story-driven experience like the original, boxed retail game; it's a different experience, and one that I feel compliments the original without adding to or detracting from it.
It's more action orientated, and I love the 'challenge maps' that come bundled with it. In these you have access to all sorts of items and weapons, but must survive until dawn with no extra health or streetlight 'safe' zones in which to take a breath.
I've played through Alan Wake's American Nightmare
just the once, but keep coming back to the challenge map levels. In my opinion it's well worth the price.