Technology has made the details of Clausewitz's work obsolescent -- for example night attacks by US forces are highly effective. And strategic nuclear war approaches what Von Clausewitz thought merely an abstraction: "absolute war." Nonetheless, "On War" remains the definitive text on the unchanging fundamentals of military strategy and the relationship of force and policy.
"On War" will always be a dense and difficult work to read; it has so much information in so (comparatively) little space, and the concepts are frequently new to modern eyes. Nonetheless, a great translation makes the book far more accessible than does a poor one (such as the Wordsworth edition uses). The Howard/Paret translation is the definitive English language version. Any student of war, or even of politics, should read this book in this translation.