It's decent.
MC transmigrates into Kakashi and fuses with his soul.
Story is written well, but lots of issues with the characters, especially MC and Hiruzen.
MC : despite having the knowledge of future events and the characters, still, at times, acts like he doesn't know anything. He freaking thinks that Danzo won't kill a leaf shinobi!! He trusts 3rd a bit too much. And is shown surprised/wondering about a lot of things that are very clear in manga/anime. So it's very inconsistent to see MC using his knowledge in one scene, and act like he doesn't know anything in other. Also for some reason, he decided to write down all his memories from his past life in a freaking book! Pure stupidity (I guess in future someone will steal it to make the enemies stronger).
3rd Hokage : He is shown to be even lamer than in canon. Danzo sent root to assassinate him? Okay, no need to do anything or even mention it to Danzo. Danzo is still maintaining root? Alright, no worries. Danzo tried to kill Kakashi? That's okay, I won't ever bring it up or take any action.