CGGU verdict so far. For context, I'd read the quite thorough synopsis on the NCC website (is that the one? Somewhere on the web, anyway), and ummed and ahhed a lot before deciding to read it.
1960s: it doesn't feel as vengeful as some of the reviews said. I think it's actually pretty humorous a lot of the time. I can imagine this humour is much less funny if you don't see a funny side to Joey-hate, for example. Incidentally, one of the things that put me off reading for a while was the knowledge that two of the characters I feel warmest towards are killed off v early on. Actually, I am glad for this because I think they have been spared the kitchen sink misery, perhaps deliberately. It is a bit kitchen sink ish. (Side note: it is actually three of the characters I feel warmest towards who are killed off early on. One is totally off-stage and casual though.) Margot and Con are hugely likeable. It does largely feel incredibly true to what I think of the various characters, except for ML taking Reg. ML is otherwise perfectly in character, but I can't believe she'd take someone else's husband or, frankly, look twice at Reg.
1970s: much less keen. Very kitchen sink. Child death is no fun and actually I don't really want to read that. The off-screen plot-irrelevant early death is mentioned again and now feels gratuitous and annoys me. Margot and Con remain incredibly likeable. Not really loving Sybil and Josette's appearance although arguably it fits perfectly with their final appearances in canon, where they get deprived of their expected education to trail around Australia after Madge and then marry Australians in a kind of revenge, but then I don't like that at all either. On the other hand, there are amusing moments, eg. Miss Annersley reflecting that she cannot leave the school in a crisis in spite of Nancy Wilmot dropping hints about old ladies outstaying their welcome.
Will add more thoughts as I go along! I've definitely read better fic, but it's not dreadful. It has genuinely good moments IMO but there is just too much misery all around it. Some people love that kind of thing though. I do think it's largely tongue in cheek and sort of frustrated fan seeing the problems, rather than the work of destruction I know others view it as.