I never had a 64, but I know they were popular. I wonder if anyone made much music with that little keyboard add-on. I know the Amiga and ST got used by various musicians. I think Vince Clark used a BBC Micro to control his synths. People would experiment with whatever they had as pro music gear was really expensive.
The Commodore 64 was popular for music but it wasn't as popular for MIDI in particular as the Atari ST was.
While the Amiga was the better computer, it was more expensive than the ST and didn't have the built in MIDI interface (though you could add one) so the Atari ST ended up being a lot more popular for that purpose.