Yes, well, the book HAS to be read as it is indeed a crazy, wonderful, hilarious modern classic.
And for the film Mike Nichols does a wonderful job - using fabulous photographic techniques (long lenses, long takes, and stunning compositions).
And Alan Arkin is so Yossarian, it's not true. I now cannot imagine anyone else.
And there are some superbly handled scenes: the man-in-white in the ward, the bombing runs with pipe-smoking Arfy - wonderful... and the superb scenes with Orson Welles as Dreedle ("Take him out and shoot him!") - not to mention Anthony Perkins as the padre, John Voight as Milo, and many, many more star cameos. Great stuff.
So, at the end of the day, it's a terrific film, and worth seeing for that. On the other hand it's not quite the whole book. How could it be? So see the film for all its pleasures. And return to the book for what spawned it.
But don't spurn it. It's great film-making, and Alan Arkin IS Yossarian....