The DLC comes off as a side story that sets up Alan Wake 2 basically. I'd say only buy The Writer if you really liked the game, since it's short and not really worth $7.
My biggest problem with Alan Wake is how the characters and story are apparently so important to the game, but in the end I couldn't give a shit about anyone -- especially Wake's Mary Sue wife. Barry was entertaining at least.
Just as a general rule, I measure DLC purchases in coffee. I drop a little over 3 bucks on coffee without even thinking about it. I'll buy a big latte and a scone, and suddenly I'm dropping 7 bucks on a snack. If a game has DLC, and I think I'll enjoy it more than a coffee and scone, I'd buy it.
1200 or 1600 spacebucks gets me thinking about how much coffee I could be drinking instead of playing the DLC, but those 400 and 800 point purchases are pretty easy to swallow, so to speak.
Whenever you buy DLC, you're putting a larger percentage of money in the publisher/developer's coffers than a retail purchase. If you like the game, it's another way to make sure the dev stays in business. IIRC Alan Wake didn't sell as well as everyone hoped, right?