I constantly see reviewers praising this series saying that it is the best thing since sliced bread.I find this series to be highly overrated. I get irritated by forums that constantly whine and bitch about how "original" this series is compared to more modern day shonen with little to know explaination.
To begin with the characters feel wooden. Hiei is the biggest offender. I think of Hiei as a stray dog that only needs to take one punch to be housebroken. Seriously, he gets beaten once and then joins Team Yusuke and never commits any morally ambiguous action again. We later learn about his very convoluted back story that contradicts the logic of his first appearance. This is made even worse when you consider how easily Yusuke ( whose girlfriend Hiei abducted) and Kurama (who was stabbed by Hiei)seem to trust him implicitly.
Yusuke starts out fun and snarky but his character devolves into an adrenaline junkie as the series develops. In fact by the end of the story he turns into Goku from Dragonball Z, enjoying fighting but thinking everyone can be friends.
Kurama is also an interesting charcter at first but never grows in any direction, his character growth remaining stagnant throughout the story with several chances to have the spotlight focus on his struggles ignored. Didn't Kurama lose his humanity in full demon form? Not important. Kurama met his partner in crime from his previous life, did it really matter? Not really. Kurama sacrifices half his soul with Yusuke but niether are affected by this action. In fact, Kurama doesn't actually seem to be close to them in spite of risking his life for them. He feels more like a coworker who just happens to slay demons with the rest of the team rather than a friend.
I won't talk about Kuwabara because most people including the author don't care about him seeing as he doesn't get much screen time and is kicked out of the final arc; but I do feel he was the best developed.
The arcs range from decent to downright terrible. Many of the fights are downright slugfests with power ups scattered between punches. The series also contains some pretty poorly implemented plot twists like Sensui hating humanity because a few humans torture demons or an entire military coup happening offscreen!!! Honestly, if this was the best shonen had to offer I would quit manga altogether. What a disappointment!