Product review (5 stars): a bad fuse, thermal switches
I had a dead GE Microwave and after replacing the Magnetron, it worked worked perfectly.  Replacement was simple.  Two words of warning!  There a number of other possible faults that could cause failure:  the controller board, switches in the door hitch, a bad fuse, thermal switches, a bad rectifier diode, etc.  Check all of these first.  Second warning!  Very high voltage!  I am an engineer who has worked with high-power transmitters; this is not a job for a novice!