These things were popular through the 80s until the mid 90s when CDs became the new medium. Then it became "DISCMAN" instead of "WALKMAN" and then SONY had something called the Mini Disc that never really took off. MP3 Formats then went onto digital memory cards, and Sony's Memory Stick proved too expensive for the marketplace to handle despite their elegant and modular design. Furthermore, SONY never really had a successful ecosystem, despite the fact they could've built one since SONY's subsidiaries were heavily involved in the music and film businesses.
Then APPLE came, launched iPod due to the genius of Tony Fadell, along with an iTunes ecosystem, which was met with somewhat approval from the music industry. This ecosystem of secure music and secure payments would ultimately make APPLE the world leader that the record labels would finally accept. And combined with the white, modern, beautifully styled iPod, launched by the late charismatic perfectionist Steve Jobs, the iPod took over the world by storm. With clever marketing, a clever design, and a difficult push into retailing music and videos via iTunes, Apple achieved what was once thought as impossible.
The next piece you have to add is a first generation iPod to your collection!