Keep in mind that there are already Two episodes of DLC announced. These expansions could extend the game as much as 3 hours. so while you might be finished with the game you might want to hold onto it for these two DLC packs.
Taken from Teamxbox.com:
Grab your flashlights and stock up on batteries! “Alan Wake” is available in stores today in Europe, Asia and North America, and is due to arrive in Japan on May 27, exclusively on Xbox 360. Hailed as one of the most anticipated games of the year, “Alan Wake” invites players to experience the suspenseful narrative, cinematic action and chilling cliffhangers as the mystery unravels with each episode.
Your investigation in Bright Falls doesn’t have to stop there. With every new purchase of the standard or Limited Collector’s Edition, “Alan Wake” fans score a token to download the title’s first Game Add-on episode called “The Signal.” Hold on to the token card until July 27 to redeem the exciting next chapter of “Alan Wake,” only on Xbox LIVE.*
Taken from Joystick:
"The second downloadable episode of Alan Wake will be called "The Writer." The description of the content, which recently popped up in the game's "Downloadable Content" menu, reveals that the tortured author will find himself in "strange circumstances" (surely he must be used to those by now) and must face "one increasingly nightmarish situation after another." According to the listing, the episode will cost 800 Microsoft Points ($10). The first episode, "The Signal," will cost the same when it launches on July 27th, though it's a free download for those who purchased a new copy of Remedy's thriller. "
Kevin