I think "Civil War" is a great story on its own, despite a lot of its character motives requiring a level of explanation. Fortunately, some of that exists in its supplemental material. There is some truly great work outside of the main series, be it with added background, added drama or angles you hadn't thought of. Unfortunately, some of the tie-ins are pure rot. You're about to get examples of all.
"Born Again" by Frank Miller and David Mazzucchelli doesn't re-invent the wheel. It doesn't redefine art and pacing like Miller's work on "The Elektra Saga." It doesn't carry the immense popular cultural significance of Miller's "The Dark Knight Returns" or Miller and Mazzucchelli's "Year One." What it is, though, is a freaking amazing story. In my opinion, it's the best thing Miller ever wrote.