
The reason the concept has such a stigma is that it seems to 'fly in the face' of
James Clerk Maxwell figured that light should exert a force on any surface it hits way back in 1871 (clever bloke, though this is something most A-level physics teachers will probably still deny in my experience). The trick is that microwaves bouncing end-to-end of an almost cylindrical metal 'cavity' are caused to exert unequal forces on the end surfaces, as the narrower topology of the cavity towards one end restricts them to move slower, imparting less momentum to that narrower cylinder face.
Basically, it all involves such complex physics/mathematics, that the whole concepts tantamount to witch-doctory, even to other microwave engineers (let alone the average Joe), so there are obviously some opponents of it's validity. Shawyer's tabletop size experiments demonstrate a measured force of up to 300 millinewtons (about equivalent of a 30 gram weight) using your standard power 1 Kilowatt microwave generator (as found in your kitchen). This is tiny, but highly useable in the space industry for satellite position for example. Currently half the weight of satellites is 'reaction mass' for the thrusters used to keep the device in a stable orbit until it runs out. With launch cost still around $10'000 per kilogram, this technology could replace all that bulky fuel, saving billions in a few years.
Then there's the prospect of higher efficiency "Emdrive"s: the current technology is highly inefficient; the metal walls of the cavity dissipate the microwave energy off as heat fairly rapidly, before the rays can bounce off more than an average of 50'000 times. Shawyer hopes to use superconducting materials (already commonly used and well studied in particle accelerator magnets) to massively improve this performance within a couple of years, allowing an equivalent apparatus to produce forces strong enough to lift a car (in principle) using the same energy input. In my opinion this should tie in nicely with recent advances towards room temperature supercontuctors.

This kind of potential technological revelation takes me back to the heady days of young ‘teenagedom’ when I was convinced I’d work as a scientist to develop FTL (faster than light) travel technology. I’ve long since shrugged off the desperate need for such technology, with the idea of an inevitable, technological Singularity: the product of an isolated Earth, negating the need to “boldly go” (as it’s incorrectly stated) anywhere but the infinetely more interesting future, here! That naustalgic feeling is partly why I anticipate disappointment with this development.
Also, trying to imaging a vehicle *actually* hovering there, with no measurable support seems unrealistic, especially knowing that no energy was being used to actually propel the device (all input energy lost as waste heat)! However, something as simple as a patio widow might seem unintelligible to a person before the common use of glass, let alone video screens/helicopters ("Any sufficiently advanced technology is indistinguishable from magic.") . And no energy need be input to a rope supporting a heavy mass in order that it maintain itself against gravity; there is merely the need for potential energy stored as tension in the rope’s strands. Perhaps that is a better way to view the nature of the force from this drive.
General info links: 1 | 2| 3
General info links: 1 | 2| 3