If you need a solar cell array to collect energy to produce the microwaves, the farther you get from the sun the less likely you'll collect a sufficient amount. So, for satellite positioning, sure. Looks like a great idea. But if you're looking at interstellar travel, I think you'll need an atomic power supply. That might do the trick.
Shawyer's engine is extremely light and simple. It provides a thrust by "bouncing microwaves around in a closed container." The microwaves are generated using electricity that can be provided by solar energy. No propellant is necessary, which means that this thrusters can work forever unless a hardware failure occurs. If real, this would be a major breakthrough in space propulsion technology.
Obviously, the entire thing sounded preposterous to everyone. In theory, this thing shouldn't work at all. So people laughed and laughed and ignored him. Everyone except a team of Chinese scientists. They built one in 2009 and it worked: They were able to produce 720 millinewton, which is reportedly enough to build a satellite thruster. And still, nobody else believed it.
I remember the old Voyager instellar space craft has a radioisotope generator onboard it.
But hey, I'm no rocket scientist.
This youtube video shows a benchscale model propelling the unit via microwaves.