Regardless of my displeasure with the new "creative" casting choice... it looks like the actress is going to be a great character/villain, even if it's not really the proper Master character that we know from Delgado and Ainley (Simm wasn't The Master either). It'll be cool if this time "she" sticks around to trouble the Doctor once or twice a season like "he" used to, instead of just turning up in a special and dying.
Oh, and why is she dressed exactly like Mary Poppins anyway?