Before Apple dominates the mp3/mp4 market, I specialized in mp3/mp4 product integraion, and have enabled 5 millions+ players to the world.
In the old days, as a young engineer, developing consumer electronics gadget was challenging. You have to evaluate the best development platform, assist hardware teammates to design schematics, board bring-up prototype, code your firmware with power consumption and RAM/ROM memory limitations, integrate connectivity (e.g. USB, bluetooth, BLE and etc) with the counterpart system.
I was lucky to be given the chances to work on new platform and new technology. Although it was very challenging, it built up my knowledge in embedded system.
If you ask me which mp3 player I like most, I would like to nominate Sandisk Sansa Express and PD-03. The mechanical design and form factor is perfect.