I've just finished the series and quite enjoyed it.
It's nowhere near as good as the movie, but there are some really funny lines and they've changed the storyline just enough that it isn't entirely predictable. The anti-SJW stuff that upset the critics has gone by the end of the first episode and is barely even mentioned as the series progresses. They've also kept the original ending.
Aside from that, it's gleefully un-PC, and as bitchy as bitchy can be, especially the teachers' meetings and the war between Heathers Chandler and Duke, which is the most enjoyable aspect of the show. It could have been written by a Datalounger.
It's not all good, however. The actress playing Veronica is extremely bland and has this annoying, whispy voice with vocal fry, while the character is far more batshit than she was in the film but then goes through this Damascene conversion that doesn't really come off as believeable. She's not Winona.
Meanwhile, the actor playing JD has no presence whatsoever, and they keep giving him these long pop-psychology speeches that he doesn't have the charisma to carry off. He's basically Shawn Mendes throwing a teenage tantrum and a far cry from Christian Slater. It's a shame they're the main storyline.
The show also looks kind of cheap; the sets are a little underdressed and there never seems to be many extras around. It doesn't really detract from the enjoyment though.
The whole thing reminded me of Insatiable, although much funnier, so if you like that then you'll probably like it.