It certainly looks authentic and US officials are being tight lipped about whether it is the missing drone that was lost last weekend.

Any way you look at it, not good news. New York Times:

On state television, the video clip was narrated by a voice saying that Iran's Revolutionary Guard Corps and army had collaborated in a "sophisticated electronic attack" to bring down the plane. The unidentified narrator gave the drone's dimensions as 26 meters (about 85 feet) from wingtip to wingtip, 4.5 meters (15 feet) from nose to tail and nearly 2 meters (6 feet) in height. The narrator also said the aircraft had "electronic surveillance systems and various radars" and was "a very advanced piece of technology."

In what appeared to be an attempt to explain the aircraft's undamaged appearance, a Revolutionary Guard Corps commander, identified as Brig. Gen. Amir Ali Hajizadeh, said in the video that the drone "was detected by Iranian radars as soon as it entered Iranian airspace and was brought down by Iran's military systems with the minimum damage possible."

American military experts have cast doubt on Iran's claims precisely because of the drone's ability to avoid radar detection. "It would be almost impossible for Iran to shoot down an RQ-170 because it is stealthy, therefore, the Iranian air defenses can't see it," Loren Thompson, an analyst at the Lexington Institute in Arlington, Va., told the Military Times newspaper. "Partly for the same reason, it is exceedingly unlikely that they used a cyber attack to bring down the aircraft."

Russia and China are salivating at the opportunity to get a look at the technology and there is little doubt that Iran will oblige them.

As for how it came down, I've seen speculation that the craft may have lost contact with its base and eventually ran out of fuel. The Iranians may be hiding some significant damage the craft suffered when it came down.