I have, like many more, done the air test with each new detector I have been fortunate to own or borrow, the air test does give you some indication as to the capability of the detector, if when testing in the air you are not getting a great amount of depth you can assume the same will be true once out on the fields, you also have the opportunity to tune, adjust or program the detector to help increase its performance. You also get the chance to hear the different tones made by different metals so you are not going in blind, you have a small amount of knowledge as to what to expect in the field, like the tone for coke and hot rocks, if the detector has that ability to differentiate.
I'm not saying every air test is to be believed, but by doing your own or watching someone who's not sponsored by a brand you can get an idea as to how much of the hype to believe, especially if your looking to buy your next detector, it's up to you if you take what people say as gospel, but the air test can be a good place to start looking, then watch what results are coming from out in the field videos, if your new to detecting these 2 types of videos can help you make an informed choice as to what you spend your money on, if your a seasoned detectorist then you can always skip watching the air test videos, as you are probably already aware of the capability of the detector being tested.