I was kind of dreading this, but it is now my favorite Emma adaptation. Much better than either the Paltrow or the Beckinsale version.
Romola Garai was as close to perfect an Emma as I could imagine. Jonny Lee Miller was an excellent Mr. Knightley. I adore Jeremy Northam but really he was almost too dishy to be a credible Knightley. With Northam around how could Emma ever think herself in love with anyone else? With Miller, Knightley became a more credible character -- that pleasant, cultivated, somewhat older man from next door that Emma had known all her life and never realized she loved because she was so used to him.
Michael Gambon was wonderful as Emma's father. He is easy to overplay to comic effect for his idiosyncrasies -- a foolish, fussy hypochondriac. With Gambon, he was more nuanced -- his fear of illness and accident was understandable (people did die of such things a lot in the 18th century, and he had lost his wife in tragic circumstances). He seemed genuinely loving of Emma, not just exploiting her as a dutiful daughter. You could understand why Emma was genuinely fond of him.
The rest of the cast was also excellent and the English countryside never looked so gorgeous.
There were some smallish glitches. I am quite sure, for example, that Frank Churchill would never have sprawled on the ground with his head on Emma's lap, as he did in the Box Hill scene. All Highbury would have been shocked.