One thing is certain: The Declaration of Independence, while recognized as important in its own time, has grown in significance and renown as time has passed. On one level, its significance is obvious, for the Declaration reframed the purpose of the Revolutionary War to include independence from Great Britain. This of course led to the creation of the United States as a sovereign nation, and was thus a crucially important step.
More important in the long run, however, have been the ideals expressed in the Declaration, which have become a sort of statement of faith for Americans. The statement that "all men are created equal," easily the most quoted passage from the Declaration, has been a sort of touchstone in American political rhetoric. Abraham Lincoln drew on this phrase in the Gettysburg Address, as did Martin Luther King in the famous "I Have a Dream" speech. King in particular used the line as an aspiration, something Americans could one day hope to achieve. At the Seneca Falls Convention of 1848, Elizabeth Cady Stanton used the document to illustrate the ways in which women were oppressed despite having the same unalienable rights as men.
In short, the document has come to be viewed as a statement of American principles, and despite the historic failure to live up to these values (even the author himself, Thomas Jefferson, was a slaveholder) it has inspired revolutionaries around the world. French and Haitian revolutionaries, and even Vietnamese leader Ho Chi Minh explictly referenced the document in making claims to liberty. So the the Declaration has become a crucial text for those asserting human rights and liberties, and this is its enduring importance.