So after a using rockbox on my iAudio X5 I must say that I am impressed, and I'll likely stick with the Opensource Firmware, instead of the Vendors'.

Battery life with Rockbox is slightly over 6 hours. On a one year old X5, so not on the L version with extra battery life. I obtained this value using the battery bench plugin, and playing shuffle over my whole collection. I've heard that for "vendor benchmarks" they usually choose 128 kbit mp3; my collection is mostly higher bitrate, and Ogg Vorbis is also more CPU intensive AFAIK. Crossfading also takes extra CPU cycles, for a fair benchmark I would turn that off, but I wanted to know this value for my everyday use.

This very likely is less than with the original firmware, but enough for me. In fact, I even forgot turning off my player in the afternoon, and the battery still held until I got home. And like one extra hour.

Sound quality is excellent, but I'm no audiophile. It's just that I really like the way it sounds. Although I couldn't hear any difference with the EQ turned on or off (is it already implemented?). But I don't care. ;-)

I managed however to crash the firmware once or twice these days in the menus. Can't really say when, only that it was when going to the system settings from some file properties or so. But it's still beta, so maybe this is already fixed. ;-) Guess the crash did cost me a few extra minutes of battery life.

There is some oddness in the navigation due to the wide range of platforms supported. For example the battery benchmark app claims you need to press "play" to start, but that is in fact clicking the stick, not pressing the real play button on the side of the display.

Having the player display the next track in queue also was nice: when I didn't want to listen to a certain track I could sometimes skip the next one, too.

Display poweroff is also nicer with rockbox - the Cowon firmware used to low-light the display first, then turn it off completely. That seems pointless to me, and annoyed me a couple of times, actually.