Mostly true, yes. There are very few films I have seen, having read the book, that didn't in some way disappoint. Maybe a scene that was in the book, that didn't make it to the film. Perhaps a character that "feels" different on screen from what it was between the pages.
Most times it's just that the book is too long for a 1.5 to 2 hour film to do justice to. That's why some short stories work better as films IMHO.
You can actually use Stephen King stories for examples of all of the above;
Lawnmower Man - no resemblence to story - awful
Salem's Lot - follows story for most part, but swaps physical characteristics of the vampire and his familiar - which doesn't make much sense.
Shawshank redemption - follows the book almost exactly and is excellent. One niggle. When they tar the roof, and Andy tricks the guards into bringing them all a beer. In the book, the beer is warm (the guards revenge); in the film, it's icy cold. A little thing I know, but it still niggles!
Ian