The moment that worked best
When we get to see the other side of the mirror. I like the concept of an empty dark space as presented before the brother's girlfriend's doppelganger came out. I liked the skittering footsteps and the malicious whispers of the doppelganger, angry and ready to make it's move. I wish we could have gotten more of this "otherspace", like a tour through the brother's apartment from the mirror side, negatively lit rooms with occasional windows into our world, positioned exactly as the mirrors are placed in the apartment.
Long pans with no relevance to the scene were the worst parts for me, and really worked to take the viewer out. A few more looks at the faces of evil that the mirror others could portray would not have hurt. Perhaps someone grinning as they drove the other car during the crash.