How bad is it? Der Spiegel calls it a "meltdown" for U.S. foreign policy:
251,000 State Department documents, many of them secret embassy reports from around the world, show how the US seeks to safeguard its influence around the world. It is nothing short of a political meltdown for US foreign policy.
The New York Times is a bit less excitable and merely refers to "brutally candid views" and "frank assessments" that have now been exposed:
A cache of a quarter-million confidential American diplomatic cables, most of them from the past three years, provides an unprecedented look at backroom bargaining by embassies around the world, brutally candid views of foreign leaders and frank assessments of nuclear and terrorist threats.
Wikileaks is tweeting tidbits about the leak and reactions to it, using the hashmark #cablegate.