I still have my C-64 (with two 1541 Floppy drives). I still use it from time to time as a controller - thanks to the openness of the design.
If you bought the (extra) Programmers Reference Guide, between it and the standard manual you had complete electronic schematics and a complete memory map - including the kernal and Basic interpreter. It was designed so that you could modify any part of the OS, modify or add to the interrupt routine, etc. Commodore went well out of their way to make that platform easy for 3rd party software and even hardware devs to work with.
A machine shop I worked at had a closet full of the allegedly "portable" version - the SX-64. They used them for a controller on an automatic surface grinder, so they bought up a lot of them to ensure they had "spares". I'll add that this shop was owned by a family, and one of the sons was a programmer. Our workstations (where we wrote the programs for the CNC machines) were Amiga's running custom software.
At the time it came out, it's competitors were all much more expensive, "greenscreen", and had no sound. Same with the Amiga, well ahead of it's time with a math co-processor (optional on the IBM) and dedicated graphics ship (which no-one else had).
It's truly unfortunate the the management was so incompetent and ran the company under - it was the best of it's time as far as design and usability.