Back in the early 80's they were the latest 'must have' gadget and it became fashionable to have microwave dinner parties, went to one where the host did microwave roast beef, I still remember that for all the wrong reasons, what a waste of a good lump of beef.
My other pet hate was ordering in a restaurant and hearing the bell 'ting' just before your meal was presented to you, invariably it was not a good experience.
As for how early, we were using industrial microwaves for vacating air from vacuum tubes when I started in the electronics industry in 1964, we used to put our lunch in there too to heat it up but there was no power control, turntable etc and gthe results were very hit and miss.