Kitty's change from vain society girl to selfless and compassionate wife is plausible enough thanks to the Naomi Watts, who's easily gives the best performance here. I've reservations about whether Edward Norton was the best choice for the role of Walter. Can you really suspend disbelief enough to think he fits Walter's character. I also think he affects the English accent in the way many non-English actors do; very slow and deliberate speech, making sure every syllable is properly pronounced so that it sounds forced, but maybe it's a churlish point to make. Not to say Norton doesn't equip himself well, and it sounds as if he was passionate about the film being made, so kudos for this.
The film certainly looks great; the use of locations and cinematography are superb, though one wonders whether the film makers had seen how Chen Kaige and Zhang Yimou had managed to capture the Chinese landscapes on film previously to make them look exotic and dazzling. Whether one gets more from the film having read the novel, I don't know. Most of its audience probably won't have read it so will approach this film as a period romance of the classic kind (it's still superior to most contemporary middlebrow film making however), whereas those familiar with the novel will probably have to judge the film next to it. It's as good an adaptation as one might have hoped; obviously things have to be condensed and simplified in cinema. I'm not sure we get the overview of the Fane's marriage that the novel presents, nor the motivations behind Walter's suicidal decision to take himself and Kitty into the middle of a cholera epidemic in China, but I think the film makers did more than enough with the constraints they had (there was also the pressure of Chinese commercial investment in the film that led to some minor cuts).