I'd give it a solid 9. In many areas the game is sublime, particularly the lighting and the atmospheric effects. Never before have these elements been realised so succesfully in a game. The gameplay itself isn't anything new or revolutionary, but it works and remains entertaining throughout the course of the game.
My major problem with it (surprisingly to some) is the storyline. I like that the story that the writers are trying to tell is ambitious and they should certainly be commended for that, but in the end the story is one big convoluted mess. At no point in the game do you have any idea what's going on, what's real, and what isn't. Usually I like high-concept plots, but they have to have some kind of resolution, and the writers didn't even attempt to provide that in Alan Wake. It channels all of the worst parts of shows like LOST.
Usually I can enjoy a game if the story is slightly disapointing, but seeing as the narrative was supposed to be the major draw of Alan Wake, I couldn't help but feel a little disappointed by it in the end. The universe is certainly interesting enough to merit a sequel though, so I'm sure these problems could be rectified if such a follow-up were to be made.
One more thing. Because the atmosphere and lighting is so brilliantly executed, it was a bit disapointing that the only enemies we fought were humanoids, birds and inanimate objects. If a little more thought had been put into enemy design the game would have been truly petrifying.