I love clip and rockbox. A while ago I compiled for someone of why I use rockbox:
Audiobooks:
- faster speed playing, including correcting the pitch
- auto stop/start with device power / headphone unplug.
- bookmarks, autobookmark on stop, recent bookmarks
- auto-rewind on playback
- dynamic range compression (good for noisy environments, in the car, etc)
I just leave it in the car and plugged in to the charger. When I turn on the car, it rewinds it a set amount and begins playing.
Other:
- advanced parametric equalizer
- fast forward tweaks (accelerate faster, slow down near beginning/end)
- ease of playlist manipulation (add this album / folder to end of my playlist, shuffle just this album or whole playlist)
- advanced crossfeed
- last.fm log support
- displaying more information, customizing the display
- recording customization (bitrate, stereo, etc)
- reading menu options / artist/song names so I can use it in the car without looking
- balance (with certain headphones, the balance is off for my ears)
- karaoke mode (for getting rid of some crappy metal vocals with otherwise good music)
- games (ok, now that I have a smartphone, I don't use this anymore)