Black Christmas did indeed suck. Hard. However even in that I LIKED Mary, she's a really winning presence and can make a strong impression in even a small part (Death Proof).
I particularly loved her in Die Hard 4.0. Her tough cookie performance as Lucy McClane hit exactly the right note in a role that could easily have just become a damsel in distress part. I'm not sure she's the world's most gifted actress but she brightens up any movie and that's enough for me to alter my previous Undecided to an unequivocal YAY.