I remember when I got my Saturn and Playstation, I played the heck out of the pack-in demo discs. The Interactive Sampler 8 with my PS1 easily convinced my that I needed to own Metal Gear Solid, but oddly enough caught me on NFL Xtreme as well. The Bootleg Sampler that came with my Saturn piqued my interest in The Mansion of Hidden Souls. Later, the Metal Gear Solid 2 demo with Zone of the Enders hooked me for that game, and the Santa mission of Toy Commander with an Official Dreamcast Magazine disc got me on that one.
Any good stories of when a demo did its job on you?