Sorry, no great insights or massive revelations but would like to say thanks for Rockbox.
A couple of observarions:
1. User interface. Find it a bit more complicated than either of the original firmware on the Clip+ and M5L. Bit of a pain to start with but finding my way. If there isn't already (I'm a techie, I don't RTFM

) perhaps there could be an idiots switch sometime? Something which means only a much simplified UI is presented but has the option to unlock the more complete one if people wish.
2. My M5L despite having the latest firmware locked up on occaisions requiring the pin reset. So far nothing like this has occured with Rockbox so thumbs up!
3. Volume seems to go higher than the manufacturer's ones which is good. Have had a couple of occaisions on trains where the background noise blots out the music. Also if using my older Sennheiser HD530s the volume could be a bit too quiet, which is no longer a problem.
4. On the subject of the M5L have had a couple of occaisions since the upgrade where the switch on switched off. What I mean is when I have switched the device on the screen lights, then after a second the player switches off again. However if I switch on and hold the power switch until the boot screen is displaying it seems to boot up fine. Never noticed this with the original firmware.
5. Music quality from both of my players seems more "crisp" but as I can't do a side-by-side comparison that could be subjective.
6. Lastly I am hearing loads of tracks I never tended to hear before. By this with the manufacturer's firmwares I used the shuffle play modes. With Rockbox I am using the History never played mode and so far seem to be hearing tracks I don't recall hearing previously. Can only guess you're using a different randomiser algorithm which is picking different tracks from what the shipping firmware would have done.
Overall very well done, am suitably impressed with the quality, polish and ease of use. You guys should create your own audio players, bet you could teach the big boys a thing or two.
Cheers.