Three *** is probably over-generous! I actually only watched the movie because I like Keira Knightley. She was lovely - but she really didn't have much to do. In the book, the character of Helene is rather ephemeral - we aren't supposed to know what or who she really is until the end. The same is true in the movie although the script does attempt to flesh out her character somewhat.
However, this flick literally did send me to sleep. It is v-e-r-y slow and is made worse by the narration (by the lead actor) which, even when it SHOULD have expressed excitement or drama, was simply monotonous. More than anything, I think Michael Pitt's lack of vocal expression was the thing that made me doze off! I had not seen (or heard) Michael Pitt before, and I'm not holding my breath to see him again. I think he was actually miscast. As Herve, he is chosen to go on a hazardous expedition from France to Japan. This selection takes place fairly early in the story and, right there, it lost me. I could not/cannot understand what it was about Michael Pitt's "Herve" that made him a likely candidate in anyone's mind for such a venture. Not even a spark of adventure spirit here! Frankly, I didn't think he would have made it to the Belgian border - let alone all the away across the Russian Steppes, through China, then sailing to Japan ("sort of" in disguise since foreigners were not permitted to enter Japan at the time). Then, whether meeting a bandit/shogun type of character OR being fascinated by the man's concubine, both his facial and vocal expression hardly changed. The acting was just like the narration....flat!
Having said that, visually the movie was quite beautiful at times - and the soundtrack was OK. However, on the whole, I was obviously not bowled over and, Keira or no Keira, I'm sorry I wasted my money on this one.