Tense and very enjoyable! It reads pretty literary fiction-y, but the plot then takes a quick turn towards genre. I like both literary fiction and genre, but they don't always make an easy marriage (not even talking about Nick and Amy!). My rule of thumb is that is that literary fiction could be about any of us — the art is in the telling. Genre is usually about a situation so outside most of our experiences, that it becomes pure escapism (there can be a lot of art in that too, though!).
In this case, if an unreliable narrator is a turn in a screw, Flynn gives it another turn by having two very unreliable narrators. It's also a little bit "Real World: The Marriage Edition" as in, "we find out what happens when people stop being polite and start getting real." If you're a sociopath, the "getting real" gets scary fast. What I also liked was the non-sociopath is just as sick in his own way, and just as unreliable a narrator. Flynn shows how no narrator (except maybe teenage ones) wants to tell the reader all the gory details about themselves. I didn't always find everything in Gone Girl believable, but it's a very fun, almost breathless read nevertheless.