Its very old technology - basically a camera CCD detector minus the infrared filter.
Like all devices of this type, at short range they can work quite well as long as there is some ambient light - add an IR illuminator and you get a good sight picture out as far as the throw of the light source.
They have some good points, they are rubberised, very robust and more than water resistant. They also have quite good optics at the price point a good digital zoom, and can take photos and video in both B and W and colour - stored on a removable 32 gig storage card. You can also send output live to a monitor (depends on model).
They were primarily designed for surveillance (and have been out for a very long time), were popular with police departments and others that had that requirement.
I have one which travelled around in a back pack for a longtime and it never let me down. However in the digital realm, they have been surpassed by products like the Sionyx Aurora and Aurora Pro which can be used passively at much lower light levels and across a broader spectrum.