Well, I've just finished reading this in the last couple of days. I've never seen the film, although having now read the book I suspect it is probably better than the book.
Most people will have a good idea of what the book is about either because they have seen the film, or more broadly, just by the notion of knowing what a catch 22 situation is.
I intend to watch the film, but I am also one of those people that will always read the book first if the opportunity to do so arises. The book itself is long, clocking in at nearly 600 pages and I am not convinced that it really needed to be that long or that the book is the modern classic it is held up to be. I don't propose to undertake a detailed synopsis here, those looking for such a guide can look to wikipedia or google (http://en.wikipedia.org/wiki/Catch_22). What I will say is that the book is very funny at times and I liked its subversive satire. On the other hand the books is essentially one joke over and over again and in many ways reads like a collection of short stories or vignettes shoe-horned into the medium of the novel. The plot is difficult to describe, simply because I am not sure the book has a plot, so much as a sequence of events which illustrate and repeat the catch 22 theme in a fractal-like pattern. For all that the (open) ending is genuinely uplifting and the central character, Yossarian, is one of the more accentric lead characters to come out of Americal literature.
My overriding impression is then of a book which is, at times, interesting, funny, unique and ground-breaking; but which is also long-winded, repetitive and ultimately hard to warm too.