True *almost* 100% of the time
As much as I love films (and I really do), those that were based off of books or short stories are rarely superior to the text, even in the case of a really good movie or series. The adaptation just doesn't come close—usually. (In a handful of unfortunate cases, both the book and the flick were equally shitty). I don't think this opinion is limited to people in my profession either; I've heard it expressed frequently enough by people across a wide spectrum.
My inability to avoid printed page to silver screen comparisons produces regular disappointment. Consequently, in recent years I've tried to watch the movie first if possible. This worked very well with Gone Girl, for example, which I was able to fully enjoy before savoring the book afterwards, even after knowing the shocking twist that lay ahead.
I think I'll write a follow up post soon(ish) about movies I actually enjoyed more than the book. Off the top of my head, LOTR (Lord of the Rings for the non-Tolkien fans among you) would surely make the list, as would Bridget Jones's Diary. What else would you add?