Electricity from walking

Can you believe that it's possible to produce electricity from walking?

Here are some videos that shows how it is done.

The first link below is an instructable about how to install under-floor power generation using piezo (contact) microphones...

The second is a video about piezo microphones that can be installed into shoes to make portable charging devices...

But wait! Is this really as good as it looks? Can we really generate a useful about of power just by walking? This video breaks down the maths...