Absolutely amazing series, one of my all time favorites. What an experience. The art is rough but fluid and beautiful, the character designs are great, the story that unfolds is heightened by every single character being likable. Seriously, there's almost no wasted potential when it comes to the cast across the entire series, everyone is awesome. I think Samura did a really good job balancing Manji and Rin's journey with that of the Itto Ryu. Because of perspective switching and great writing, you care for both sides. Anotsu and his whole gang are just as engaging to follow as Manji and Rin are. Add on to this that the series has some of the best fight scenes in manga as a whole and you can see why it's highly praised.
Blade of the Immortal isn't especially complex in its story or themes. It offers a simple plot structure but with depth derived from how strong the characters are and how engaging the scenarios tend to be. There are some moments in this series where the reader will simply be in awe. I can't spoil them, but some seriously badass stuff happens. BOTI is a great manga that I recommend to just about everyone.