A recent poll of prominent Historians found that 98% of historians rated W's presidency as a failure. 61% rated his presidency as the worst of any president. 35% rated his presidency in the bottom 10...many of those ranking his presidency only above James Buchanan.
From George Mason University History News Network:
With his unprovoked and disastrous war of aggression in Iraq and his monstrous deficits, Bush has set this country on a course that will take decades to correct. When future historians look back to identify the moment at which the United States began to lose its position of world leadership, they will point—rightly—to the Bush presidency. Thanks to his policies, it is now easy to see America losing out to its competitors in any number of area: China is rapidly becoming the manufacturing powerhouse of the next century, India the high tech and services leader, and Europe the region with the best quality of life.