I love my old hand crank Biddle. If buying new today I'd go AEMC.
Match the megger to the voltage classes you work with. If you work on 4,160 motors you need a 5,000 VDC unit.
IMHO top uses for a megger are establishing trends and looking for problems that aren't obvious. Can be a time saver on shorts that come and go or are moisture related. It does require that one have an idea of what a pass/fail reading is. I would not call 100,000 ohms to ground "okay". What is okay on a 120 VAC motor isn't the same as what is okay on a 4160 motor.
Also need to know when not to use one, such as a hermatic in a vacuum.