I was about 15 and the happy owner of an Atari 520STFM. I must have had the ST for about 1-2 years and spent all my time using it. Then I visited a friend who had an A500 and showed me the RSI Megademo. From that instant onwards, I was sold. I think I bought an A500 some months later (much to my parents disapproval).
I wonder how many machines the RSI Megademo actually helped sell? I know my local computer stores often had that demo running (complete with a bunch of us kids hanging around watching it).