Phil, they are of no use for mapping a car on boost, but are vaugely useful for checking that a car is running economically at part throttle.

People sell them because people buy them, not because they are very useful.

The narrow band sensor is basically only guarenteed as reliable as a switch around 14.7 AFR

So if will show you if you are above or below 14.7 AFR but it wont accurately tell you how much by!