I've always loved the movie. The book I read once and wasn't impressed. This could well be because I saw the movie first, but I just feel the movie has its own special flavor. On the other hand this is true for me regarding other Alistair McLean books, especially
Breakheart Pass. Then again I've never read
Where Eagles Dare or
The Guns of Navarone, so I'm probably not the best judge. I liked the ending of the movie better, where the 'trick' didn't work and they had to take more drastic means. I liked the book better in that the main character was the brother of one of the Zebra scientists, but then I liked the in-jokes in the movie.
Rock Hudson: "On a submarine we're all on a first-name basis. My first name is 'Captain'."
Patrick McGoohan: "As you may have guessed, my name is not really Jones." (Of course not. It's John Drake.)
On the other hand I have to admit that if it weren't for McLean's imagination there wouldn't be any movies based on his books.