Wow. As a huge huge fan of the books this is a dream come true. I'm onto my fifth re-read and having watched the series, now three times (yes, I'm obsessed), I just love how they complement each other.
There's of course a few changes to the story but for the most part, they are done so with very good reason, no story will ever be able to go from book to screenplay without some changes.
Firstly, all the actors playing our main three protagonists are excellent. Cameron Chapman is amazing. I find it hard to believe he's never had a role before now.
Lucy's character is great in the books but Ruby does such a fantastic job that she's even better on screen. And George... well honestly he has been changed the most but Ali's acting is consistent and I found I loved his charactisation even more.
My only issue is more of a personal one, and that's the anti climatic feelings I had about Combe Carey Hall, but I completely understand this as special effects cost a fortune and it is clear showrunners are looking to spread out their budget for the bigger scenes nearing the end of the series.