Rockbox has a ton of features. Some of my favourites:

- bookmarking: you can set up bookmarking per directory automatically after you shut it down or stop the audio file (fantastic for multiple podcasts/audiobook listening)
- voice commands: especially useful when you cannot see the display (eg during sports, biking)
- shut off timer (set it before going to bed or take a nap)
- supports lots of soundcodecs
- bunch of soundsettings
- adjustable settings all over the place

One disadvantage - due to the overwhelming amount of features it can be intimidating for new users.