I've been using Rockbox in my Cowon D2 for some time now and I'm really excited about it as I am mostly an audiophile... which is what rockbox intends to deal with... no movies in Rockbox... Rockbox can also display pictures and play games.
Rockbox for D2 is in an early stage with lost of features not implemented yet... but is possible to play music so I've have had the chance of test some of the codecs that Rockbox has. Beside normal formats: MP3, FLAC, OGG, APE, WMA (not lossless). Rockbox support also M4A, APE (higher bitrates), MPC (Musepack) and VW (Wavpack) all of them I've tested and plays fine. For Wavpack there isn't support for embedded playlist, though. Rockbox manage some other formats... I've read that there are about 19 formats supported.
In my device there is the option to use a dual boot so user can load manufacturer original firmware or Rockbox... I suppose that when the manufacturer stop delivering updates to the player... I will use only Rockbox.
By what I've read in the thread "Rockbox Project (M6)" and in the Wiki, Rockbox for M6 will have to deal with 3 devices or 4 if we take in account that lots of M3 users (like me) will want to have Rockbox too: this are M6 T1/TP; M6 SP; M6SL and M3... this will require to develop 4 version of the port. This make more difficult the development than for Cowon D2, but D2 have the problem that Rockbox will have to deal with a SD slot not implemented yet in that project.
I've noticed that to use Rockbox as firmware isn't a simple thing because it has lots of options like gapless for instance, different ways to manage EQ and Playlists and lots of options suitable for audiophiles... but the interface is not so friendly or for beginners, it requires to learn of it.
I've see too that in the thread "Rockbox Project (M6)" there is lots of technical post, which in my opinion is the most important for the moment, mixed with newbies questions, which will appear more and more as the project gets developed and user start using it.
My suggestion here is to create a thread for Newbies to allow developers do their work without the need to read and answer simple questions and comments. The new thread will allow all the Newbies to ask what they want not populating such technical thread.
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I hope that Rockbox for M6SL or M3 appears early

as I have a M3 and I'll buy a M6SL only to use Rockbox... I do had a M6SP already but I returned to provider due the stutter issue, which i deeply hope that disappear in Rockbox, as I sincerely love the sound of my Meizu players.
I'll prefer M6SL because I trust more in Wolfson audio codec than the same of Philips.