I was a TRS-80 man from 1978 until the late 80's. My first was a Model I with 4K of RAM, one of the first sold in our city. I made a living writing custom software for the Mod I & III for about 6-7 years. I ported my last customer over to PC in the late 90's. In between I also wrote software using the CP/M platform.
For our students, I had a C64 and several Atari's (400's & 800's). I had one math drill program that worked only on the C64. When that machine went down, I lost the best graphics-based drill program ever written.