I think she is a very good actress and very versatile. She has great chemistry with Maggie Smith for example, their scenes are a treat to watch.
@annegirl24: Have you SEEN the scene with her and Robert when he tells her he knows about Marigold?
Downton is her first TV stint and for that she is fantastic! Everybody else on the cast has years, if not decades more experience.

No offense, but I don't think she can act. She does this weird eye/head thing whenever she talks and it's very distracting. It's like she's trying to show emotion, but she has never been taught how to properly bring it across.