No. 5 is a surreal manga, similar to a psychedelic trip where you cannot distinguish reality from illusion. The outstanding art of Taiyō Matsumoto creates this strangely charming atmosphere that you will probably never find in any another manga series.
At first, everything seems confusing, but the author slowly guides you through his eccentric world as you discovery more and more bits of the story. The character design is extremely original. Sometimes, the reading can be harder because of the non-linear storytelling, but it is still enjoyable.
More than a manga series, I consider No. 5 a piece of art.