I've used Rockbox on my iRiver H340 for over a year now (basically since the first user-targeted betas for that player), and battery life used to suck..until four months later, when they fixed that for the H340. It is a great project, and definitely the best choice for a firmware if there is a port for your player and you love your gadget and are willing to play around with the options a bit to set them to suit your taste and habits.
I started a thread about porting Rockbox to the M6:
http://forums.rockbox.org/index.php?topic=10078.0
If you would like to participate, please read the following to see what you can do to help during the first stages of development:
http://www.rockbox.org/twiki/bin/view/Main/NewPort
There is already a bit of info about the M6 hardware around: if you have more, please keep it coming!
I would love to see this happen, but it is only possible if miniplayer enthusiasts with the necessary skills or the will to acquire them devote a bit of their time and energy to it...once the port is started, I am sure the Rockbox community will welcome it with open arms.
It is a great way to learn and have fun, AND get a new firmware for everyone running on your player!