I use Rockbox on both my H340 and my two H120s as it beats the OF hands-down in all aspects (including battery life). I like it so much, I even made a financial contribution to the Rockbox project.
However, we also have four H10 5GB players in the family (we love irivers!
) but, unfortunately, I still have to use the OF on these due to the battery life issues on the H10. It's probably the same sort of situation as the battery life issue on the H300, with the USBOTG chip sucking the power. Once that was sorted out, the battery life on the H300 under Rockbox shot up and was far better than battery life under the OF. I'm sure it will be sorted eventually, and I'll be able to put Rockbox on my H10s, but for now, I'm afraid I have to stick with the OF for the much better battery life (and to a lesser extent FM radio).