It appears there are 2 rockbox firmware versions for the M3K. The older one is binary-only from a .ru site and it uses the linux kernel from the stock firmware. The newer, bare-metal version is still in development. So there's something that works today and a better version in active development.
I use the older one which uses the kernel from the stock firmware. To my understanding done by the guys who worked also on the original firmware. I have seen just a single issue, it's minor: fast forward/rewind does not work properly. It does forward/rewind, but slower than expected.
The new native port lists quite some downsides/TODO's, so I did not yet switch over.
https://news.ycombinator.com/item?id=26870940