The concept in Sword of the Stranger isn't new. A wandering samurai who killed a lot of people during the war is tormented by his past and has "Sealed" his sword and decided not to use it anymore. Without having to look far you can already find earlier animes/mangas with the exact same concept (Rurouni Kenshin is the first that comes to mind).
Still I really loved this movie and consider it a must watch, specially for people that like animated movies. The animation one of the best I've seen in recent animated movies, with great design, fluid animations and great image quality. While not many in number, the battles are masterfully handled by BONES (animation studio) with great (animated) choreography.
As I've said the concept behind the story isn't new but it's enjoyable nonetheless, I really liked the trust relationship that forms between the main character and the boy he is escorting and there is also a special connection between Nanashi and the main antagonist. In the end you will be caring for the characters.
Finally, and it should not be overlooked. The Soundtrack for this movie is truly wonderful. Composed by Naoki Sato, it's an amazing OST filled with emotion and fits the movie like ring.