A kilowatt is comparable to a space-heater or an insane gaming rig. A Tesla burns about 20kw for normal driving.
Now, a Newton is the amount of force it takes to hold up an apple (or accelerate it at 1G). So a millinewton would be the amount of force it takes to hold up a blueberry.
Garbage for anything Earth-travel related. Gravity is 9.8N/kg, so to generate enough force to counter gravity you'll need to feed this thing 9.8MW per kg.
Your typical 100 HP car engine puts out 75.6kW, so you'd need 130 car engines' worth of power to lift a small 1 kg/~2 pound object. Not to mention that the machinery needed to channel/produce that kind of energy would be so heavy that it could not lift itself.
But it's still possibly useful for space travel. Not needing propellant is huge for long-distance space travel.
That said: they don't know why it works. (If it even works, which I strongly strongly strongly doubt.) If they figure out what produces the phenomenon, it's very possible they could get the drive to be way more efficient.
To put into proportions:
The radio isotope thermoelectric generator (RTG) of the Voyager spacecraft wheighs 37kg. So let's say we can build a spacecraft using this and the EM drive weighing a total of 50kg. The RTG produces 157W of power. This translates to a change in speed 30cm per day.
Bonus question: The power of the RTG halfes avery 87.7 years. What is the asymptotic speed the spacecraft will reach?
Asking this is like asking how long it should take for a sperm cell to swim across the Atlantic Ocean.
The EMD is simply not applicable to scales where RTGs are used, much as a 5MW maritime diesel turbine is not applicable to swimming motions of a biological cell. The power density of Voyager is a hundred times lower than what an EMD ship would need.
This is simpler to calculate than you might assume - you need only find the velocity after one half life and double it. Doing that gives you 15.4 km/s.
Of course this is considering only the decay of the plutonium and none of the active decay products. I've also disregarded the decrease in mass over time.
Hall thrusters typically operate around 50mN/kW. Gridded ion thrusters a bit lower, like 30-40mN/kW.
That being said, some exotic thrusters like the FEEP still manage to be useful operating below 10mN/kW for precise positioning.
[Edited erroneous data for FEEPs and needless rant]
