I used a C64 with an 8 track MIDI sequencer called Digital Studio which was very cool but limited editing. I and a friend also wrote a pretty nice graphical DX7/TX7 synth editor on the C64 as well. It was my first MIDI computer. It was a pattern based sequencer which helped work around the limited note memory. But it had realtime recording and quantize capabilities.

I had a buddy who made his own MIDI cards for the Vic20 and C64 and had a bunch of them MIDIed together running various custom MIDI processing programs and his home made digital drum machine. That was when I realized how cool MIDI really was.