I actually bought a USED copy of Alan Wake, but the previous owner apparently didn't cash in the DLC (Lucky for me).
The code was not tampered with and worked fine for the free download.
In my opinion, it's worth the cost on the marketplace, as I enjoyed the game and this was a nice little addition.
The second DLC for Alan Wake however is currently not free. I'll still end up getting it though (aiming for another 100%. lol)