DS6 read the first Harry Potter book after I bought it for him, but didn't really love it, so I didn't get the next one. I thumbed through one of the later books at a later date, and formed a generally low opinion of the writing. DS6, DW and I all liked the first movies, but not the last several so much, as we felt the image and animation richness and quality went up while the quality of the plots went down.
The writing quality is meh, I agree. But I'd say the plot qualities actually improve in the later books (4 and up), hitting the pinnacle in 6. Unfortunately, that's also the most completely awful movie of the series. As the plots get deeper and more intricate in the books, the movies end up leaving a lot more on the cutting room floor, because movie studios assume you can't sit still for more than 3 hours. They fixed that by doing the last book over two movies, but they really should have given the sixth book that same treatment.