Freakin' awesome.
Kids who grow up with smartphones and tablets would really have no idea how impressive some of these demos are. This technology goes back to the days when computers would struggle to put more than 4 colors on screen simultaneously. The people who make these demos do it just for the love of doing it and for bragging rights, not for money.
Terms like sprites, rasters, waveform filter modulation are just gobbledy-gook to most. The artistry of these digital sorcerers is phenomenal. Consider how the mp3 format didn't even exist yet, so there was no established way of digitizing analog sound into computer bits. Yes, CD's were just making their way on the scene, but most of us were still using cassettes. For some of these demos, these wizards sometimes invented the algorithms to quantize sound - something that would not become commonplace for years, and wasn't even supposed to be possible for the SID sound chip found in the C-64. We're talking about programming worthy of a PhD dissertation. Often the scene groups would invent a completely new effect or way of putting impossible graphics on screen, through undocumented features of the venerable 6502 chip or other sheer wizardry.
Computers have come an incredible way, and I wouldn't want to change that for a moment. It does seem, however, that we've lost some of that inspired magic along the way that makes the C-64 such a special machine even today.
Of course there are mind-boggling graphics/sounds demos that will run on modern PC's written with the self imposed constraint of keeping the code at 64k or even 4k, and the competition for who can pull the most performance out of their hardware continues unabated.