[GeoffJ]

If you are a tinkerer you'll like Rockbox. If you just want to listen to audio and don't mid the longer boot up then you'll prefer iRiver firmware.
The dual boot bootloader from Rockbox will give you the flexibility to try both worlds and then decide for yourself.

My personal opinion is that the playback quality of the iRiver firmweare is currently superior, but the word is thae Rockbox Ver 3 due early May may resolve this.