Movie Adaptaions of Books

So last night I rented to movies that I had been kind of dieing to see, I Am Number Four and No Strings Attached. Now while I was very disapointed with No Strings Attached ( No Movie with Natatlie Portman,Kevin Kline,Mindy Kaling and that plot should not be that bad, thanks again Ivan Reitman for showing me that you can not direct) I have actually been quiet suprised with I Am Number Four. Going in I had heard mixed reviews. Some saying its allright movie and others saying that it doesnt fallow the book at all and is terrible. I actually have found enjoyable, and a pretty decent scifi movie, and I plan on reading the books soon, but more importantly the movie brought up a question in my head. Should the movie adpation of a book be called terrible just because it misses parts from the book ? When ever talked about, movie adaptions get sayings thrown around like, " The Book is always better" but I dont think that is the case in most cases. One prime example is A Time to Kill by John Grisham . I saw the movie before I even thought about reading the book, and I loved it, I consider it one of Mathew McConaughey's only good movies. So I tried to read the book, and i just couldnt read the whole thing .I found the movie to be ten times better. The dialouge was more realistic in the movie and Idk it just seemed to do everything better. Now I am also comeing from a life long experance of seeing directors ruin comic book films becuase they like to change major details in characters personalitys and sometimes just dont take it seriously. But I also coming from being a comic book fan, a movie fan, a novel fan, and a inspiring writer understand with novels expecally that not everything can get in. Many subplots have been taken out of the Harry Potter films and those are still great adaptions.So I would just like to ask my fellow geeks to remember this when watching a adaptaion of there favorite book, that it is a adaptaion not the book itself. I am not asking you to let big character development, big time plot develping and terrible screenwriting go, but what I am asking is to not let the small stuff get in the way of what could be a pretty good movie.