Ichi the Killer holds a special place in my heart for being perhaps the first seinen or 'adult' manga that I remember reading and enjoying for its outlandish characters; over-the-top violence; and exploration of extreme sadomasochism. Such was my teenage enthusiasm that I bought the live-action movie directed by Miike Takeshi which is shoddy in the strict definition of adaptation but is still rather entertaining if you're into surreal black comedy. At the time however I was still 'shonen' and so probably too immature to fully appreciate this work, the only other manga I'd encountered being volumes of Dragon Ball in my school library. Years later I thus re-read Ichi the Killer and found that - while enjoyable - it suffered from a meandering storyline confused in what it was ultimately trying to say. In comparison this prequel '1' is much like its title: with a shorter, simpler storyline that lacks the interesting characters or setting of its predecessor. Other for the odd fight scene it holds little to no value and I would recommend reading the original or watching the movie instead.
Ichi is a prologue to the Ichi the Killer manga and has a very different presentation from its successor. Ichi the Killer is full of gore and extremism, while Ichi tells the story and past of the character Ichi with incredibly well-designed and very exciting fights.
I have to say that it's a little confusing at the beginning but the course it takes is an absurd hype. The ending was very random and filled with edgy and gorefest content ... which I will not lie saying that I did not like it, but it gave a huge change in the focus of the manga. Other than that, pretty good read for me, the school part was incredible, Ichi has an amazing and interesting development and the art and fight scenes are fantastic.