Whilst there were some interesting observations on the engineering involved in the construction of Machu Picchu, such as the terracing, drainage, erosion avoidance and water supply and the possible reasons for the choice of location I was disappointed overall.
There were some nice aerial views of Machu Picchu giving an idea of its location, which probably make the DVD worth buying since it's only 2.99 BUT there are also a lot of sections particularly in the first half of the video which have not been well filmed at all, it is almost like a home video when someone pans too quickly and the video goes out of focus. Also quite a few blurry scenes of fictional re-enactment of the Inca life, perhaps it was supposed to be atmospheric but I just found it a little annoying. Given National Geographic's great reputation for its photography I thought this was very poor.
As I knew little about Machu Picchu I came away with a little more knowledge, but I feel the idea it was "Decoded" is a great over exaggeration. It is a video narrated by one of the slow talking "stereotypical" documentary type voices and they explain how it was discovered in 1911, some possible theories as to why it was built and why in that location. A couple of more current experts show how a few extra things have been found which tend to indicate initial theories are not strongly supported. However except for being impressed by the sheer engineering exercise with the apparent absence of the wheel or steel even in the 15th century, I felt all I learnt were possible theories since nothing was conclusively justified.
Also I don't like getting 10 minutes of adverts for other NatGeo products before I get to the screen which allows me to select viewing the 52 minute show.
Since it's cheap, if you have a passing interest it might still be worth buying but don't expect to be amazed by the quality.