The only demos I specifically seek out are ones related to games/series I love. Like how I bought the Japanese Tail Concerto demo. But I've ended up with a fair amount of demos over the years just by buying lots and what have you. Though it's still only a small fraction of what exists, I have a pretty decent sized collection of PS1 demos. They're pretty interesting from a historical standpoint, and they often capture the atmosphere of the time even more than a regular, full release does.