Always room for more discussion, but there is a section of the FAQ devoted to this very question.

The only episode I have seen that I am qualified to comment on is the "Neolithic Revolution" episode. It is far from perfect, but it certainly isn't too far from the consensus in the field at the moment. It is pretty much on par with what you can expect from a well-written textbook on the subject, which is to be expected given the huge breadth of topics covered by the series.

I second the "check the FAQ" posts, but my overall opinion: it's not bad for a populariser of history, and if you want a basic primer on historical topics you don't know anything about.

I suspect I'm not the only historian to react like this but basically, I found the episodes on topics I'm not an expert at all in to be quite interesting, while those that I do know quite a lot about seemed like ELI5 videos. Though I guess that's inevitable. Also, occasionally something he says about your specialist topics can annoy you incredibly disproportionately (my most pedantic point of irritation is his incorrect pronunciation of "Jacobins", for example).

To be fair to John Green, I think he learns from and tries to respond to criticism. In his second World History series, he did indeed try to incorporate more historiographical debates and conceptual analysis, and I commend him for that.