I've been too turned off by this episode to review it. It started out decent and then went way downhill after the face-switching scene. I think the ending was a cheap cop-out with the convenience of the off-screen 911 call and the drawing in Bob's apartment that lead everyone to the four corners. The jokes ranged from good to awful.
Overall in quick summary, this episode simply should have never even been produced. The content of the unnecessary face-switching scene is one thing to frown upon, but I'm even more upset that something like that played out in a canon episode. It's a shame how many people are calling this the best of the season, because IMO I think it's down there between the worst episode, "Greatest Story", and "Thursdays". 2/5, because it at least started off interesting and had some pretty amusing gags.