Most authors complained about it yes. Burgess accused Kubrick of not having understood a single word of "Clockwork Orange". Nabokov was the most gracious when he said that while Lolita was a very good movie, he didn't recognize his original work at all :)
I think it's a very difficult exercise to adapt a novel. Should you literally put on the screen what is written in the book ? What's the point ? You're not creating much. On the other hand, if you deviate from the original work, fans start attacking you for "betraying" the spirit of the book.
Billy Wilder told the story of adapting Stalag 17. Co-writer Blum showed up the first day with a copy of the original play and said “I thought we could use this.” To which Wilder showed him how they could use it: by dropping it on the floor and using it as a door stop.
Don't forget Dr. Strangelove, which is based on a very serious novel about nuclear war, "Two Hours To Doom" (published in the US as "Red Alert"), which Kubrick and Terry Southern turned into a comedy. (Today the book is out of print and largely forgotten except for its relationship to the film.)
The Godfather was pretty close to the novel, with much dialog line-for-line identical. The main difference was the subtraction of a few egregiously daft subplots.
Hum, there was also a lot of differences with the importance of the character of Johnny Fontane who is way more present in the novel.
In this instance, the "opposite" actually happened. Coppola is supposed to have said, when finishing the last page of the novel, "I'm supposed to make a masterpiece of this turd ?"
I think it's a very difficult exercise to adapt a novel. Should you literally put on the screen what is written in the book ? What's the point ? You're not creating much. On the other hand, if you deviate from the original work, fans start attacking you for "betraying" the spirit of the book.