It's not terrible. It does make me cringe at some parts, but that's not saying that much for the genre. It's an interesting take on high school, and I appreciate that it's told by someone who actually acts their age (even if he's in a kid's body). This plot device is more used than you would probably think, and I've seen supposed 20/30 somethings regress into kids emotionally and mentally. Thus far, that hasn't happened in this manga.
The protagonist at least attempts to be not swept away into the flow of things, and his take on the serious things is significant. I don't particularly like any of the other characters except maybe Kariu, who despite occasionally being annoying, is excused for her immaturity because of her age. Yoake has issues, yet attempts to give advice, and it's kind of painful to watch. Clueless girl is clueless. Clueless boy is clueless.
I'm torn between whether to strongly despise the serious (life advice) elements to this manga, or to appreciate them for their rarity. The problem is that the serious elements drag out humongous plot holes that even the premise can't magically solve. I like the backstory, but the backstory ruins the premise entirely: that he's a NEET chosen for an experiment.
Because, while yes, by definition, the MC is a NEET, he's not the traditionally socially-awkward, worthless, lacking major self-confidence NEET. He's pretty normal, has some baggage, and is struggling to maintain faith in how society runs through because of real-life experience. I think anyone who's lived long enough in the real world can relate to that. I just hate when the other characters, who seem to have way more difficult times juggling their own problems (which IMO, they handle miserably), seek to give him (sometimes terrible) life advice. It's painful. The MC's problems are things that can be 'solved' with a little more self-reflection and some positivity in his life. He's not so, forgive my language, ****ed up, that he needs this kind of premise, which is why this manga can be irritating to so many people. It's like, we finally get a competent MC for the trope, but the fact that he's competent ruins the story. That's more depressing than any plot development.
For now, this is a solid 4.5. It'll go down if there is a pairing for the MC with high school kid, because as we see in this manga, there's a huge (and I mean huge) maturity difference. Being a "pervert" is beside the point---I can't see the high school girls as more than children, and I can't see him forming a successful/balanced relationship with any of the potential love interests, and I'm at chapter 55. I get the whole, "she's cute" angle, but if there was any real attempt at making this a successful relationship, we would also get the, "she's pretty mature for her age" from the serious parts. But we don't.
In summary, this manga kind of funny in some spots, but it would be a stretch to call this manga good. I would recommend it to someone who specifically asked for certain tags, but to no one else. It's not something I would read unless I had time to burn.