I read books 1,2 and 5, in futile hope that "maybe the next one will be better."
You missed out two of the best books in the series - 3 and 4. I haven't finished seven yet (it's been put on hold because I can never get round to reading it) but what I have read has been brilliant so far so it could easily join 3, 4 and 5 as the best in the series. There hasn't been a dull moment.
I *seriously* doubt her writing style improved considerably after book 2 and then magically dropped off again in time for book 5.
It didn't "magically drop off" again for book 5. Book 5 was just as brilliant as the previous books. I think her writing is consistently good for all the books, it's just the overall plots that I prefer in books 3, 4 and 5. That does NOT mean I think the other books are any less good in general terms overall. They're all highly enjoyable books with great characters that I can read again and again and again and not get bored.
I didn't find any of her plots particularly interesting, either. I know what happened in books 3 and 4, thanks to my best friend and fanfic, but I just can't bring myself to care. The only character I ever liked was Snape, but only in the beginning, where he was an austere, intellectual monkish figure living on the edge of good and evil .*That* was bloody sexy. But he didn't get enough text time to keep me interested in everything else, all the kiddie travails and politics and puppy romance. And then he kind of devolved into some sort of a snivelly failed Lothario, so my interest in him died as well.
I *have* read absolutely stunning fanfics about him though. So I may not like her writing or plots, but I do appreciate the idea of the whole Potter universe. It inspired a whole lot of more capable and imaginative writers. So there's not likely to be a shortage of Snape-fics for me to read. Thanks, Rowling!