I'm actually now in two minds about what i said in the spoiler above. here's the other side of it.
Basically making the audience feel weary of him to begin with seems like a good idea as they won't be spending as much time at the end confused as to how and when he wasn't the real Moody (just to clarify, he was never the real Moody). It also leaves the audience feeling less betrayed, I'm sure plenty of people fell in love with Moody's character in the book, only to have the rug completely pulled away in the end, something that didn't happen in Philosophers Stone as we never really got to know Quirrel. In the movie, we're given red herrings, but we're also given the real lead as well before the reveal, which can be good as we're not led to fall in love with the character (well, the ferret scene makes up for a lot, but the classroom scene showed him to be torturing his students instead of teaching them)
I still leave it with it's 4/5, infinately better than the first two movies, and keeps with the same style as the third(well, almost, if we wanted that, they would have hired Curón again).