As I said with Meryl Streep, impersonations seem to be the way to go, just in general, to get someone some awards attention. But, I have a very strong affection for Michelle Williams - I think if she's not the best actress working today, she's in the running for it, the girl came from unorthodox career roots and the Academy had any sense of humility, she would have been in the running for Wendy and Lucy back in 2008.
But, My Week With Marylin is no revelation really. The film itself is nothing to speak of, it only has about 75 doomed monologues that are endlessly boring, especially the ones with Judi Dench. Michelle Williams is the film's only saving grace - and yes, she is extremely good. She recalls the spirit of Monroe almost constantly. Even resembles her in the blink of a scene, mostly in the musical sequence. She adds emotions whenever they are needed and surprised me really throughout the film.
Still, it's not a revelation in any sort of way. Williams, I think, knew her limitations within and tried her best to work on her own vision of Marylin. It's a very good performance, in a limp movie, done by one of the best actresses working today.