The Good: Great addition to the story, burning up words makes you feel like you're in the story, more combat heavy
The Bad: Extremely difficult, doesn't add anything new, rehashing of already played areas, extremely short
DLC tends to get criticized a lot for either prices, length, or the amount of content they have, and developers have a hard time striking a balance between the three. The Signal, (which is the first of the extra chapters that came free if you bought a new copy) suffers from this as well since it is so short clocking in at about 2-3 hours. Yes it's really just a 1-2 sitting play through, but it does provide a good addition to the already award winning story.
Alan Wake is inside his own story once again, but the real him is writing ways to kill himself in the stories. Why he's doing this is unclear until the very end, but each time you advance through the chapter it gets more and more violent. There aren't really any changes or additions except that finding supplies, and some times enemies and environmental hazards are hidden as words that you much burn up with your flashlight (like at the end of the main game). It's interesting and makes you really feel like you're in a story so this subtle addition adds to the experience.
Other than that there's no new characters, weapons, or anything else so just sick back and enjoy the quick little story. You do run into Barry again, but other than that there's no one else in the game which is a disappointment. Another issue The Signal falls under is backtracking areas you already played in the main game. Plus The Signal is very combat heavy and can get extremely difficult at times.
The Signal is a must play for fans of the game, but if you didn't dig the main game to much you should pass. It the game was more robust and added deeper and richer content it would be so much better, but Remedy took the safe route and stuck with the main game's ideas only.