Just finished Alan Wake and it was awesome, even if I hated most of the "action" parts. Now I'm looking at this American Nightmare thing and I'm not sure... looks like something focused especially on those annoying light>shoot parts of the game.

Should I get it?

You probably shouldn't get American Nightmare :p

The plot is a nightmare and makes as much sense as one, and the rest is action.
...though I stopped playing after a few hours. The nightmare plot didn't work for me.

American Nightmare is a lot more combat focused than the first game.

It does have a few decent locations to explore and you do get manuscripts to collect like in the first game.

I thought the plot was great. It did focus on back tracking and combat more then it should, but making a whole expansion in the tone of a Night Springs episode was great.
The narration and whole tone was really interesting and it wraps up the whole storyline at the end.

It depends what exactely it was that you hated about the action in Alan Wake because American Nightmare is very action oriented indeed. However, the action plays very differently from the AW despite sharing the "fight with light" mechanics.

In AW I dreaded every fight, in AW:AN I enjoyed them. That's because AW was a horror thriller which was focused on the tension of the events. You were running around all the time, hoping that you wouldn't be jumped by Taken. And when they did you were like "Shitshitshitshitshitshitshitshitshit!" because you knew that you'd just barely survive most of the time. So you were constantly panicking and running for the next light source with flailing arms, screaming at the top of your lungs whenever you heard a rustle in the nearest undergrowth.

In AW:AN however, the whole combat system is more fast paced, tighter. Your flashlight recharges more quickly, there's a wider choice of weapons, more enemy types with unique characteristics. If Taken enter the scene, you don't panic...you're like "Fuck Yeah! Bring it on, bitches!". And when you're done with them you feel like John McClane, Jules Winnfield, Mr. Smith and Snake Plissken all roled into one. It's pulp action, not horror thriller. It's the Old Gods of Asgard stage scene in AW turned up to eleven.

In short: If you hated the fact that it's action at all, you might want to skip the game. If you simply hated the fact that the action or prospect thereof had you on edge all the time but generally enjoy action games, I'd suggest go for it. I found AW:AN to be immensely engaging and a hell of a lot of fun.

Plus you miss out on Return by Alan Wake if you skip it, so there's that...