I have RockBox setup on my Sansa e250.
It adds a few features that are pretty cool.
1. FLAC file playback
2. Audio file bookmarks
3. JPEG image viewing
4. TXT file viewer
6. a whole bunch of games.
8. preservation of original firmware via bootloader
9. file tree browsing and playback
10. Gameboy Color emulator.
11. DOOM!
I didn’t even know about FLAC support when I installed RockBox and it is my favorite feature.
So if you have a device that supports RockBox, I would say go for it.