I loved the book, so I was super excited about the series. Admittedly, the show doesn't live up the book, but it's still entertaining. The plot deviates from book and heads in a more science fiction direction. The show is certainly worth checking out, but if you expect it to be like the book, you may be disappointed.
Books are usually turned into movies rather than series; I'm not sure how well it would work. What do you do once you come to the end of the book? Cancel the show or continue making up your own thing? As Damien has said, they have deviated from the plot quite a lot, so it can work for a TV show.
I love Stephen King but hadn't heard of the book before the show came out. I'm quite enjoying the show but am getting a bit annoyed with all my favourite characters being killed off. I have since tried to read the book but it differs so much from the show it was a struggle. I'm now used to the way the characters are portrayed in the show I think! Which is a shame.
I agree with you wholeheartedly regarding the cardboard acting. I truly hope that the cast can find a rhythm because the show has potential however the acting, and in my opinion the writing, needs to be stepped up.