I'm using Cog since years now and it's my preferred player for mp3 because of its simplicity, with just the essentials, but there is something strange about all those "little nice players" (including Play and Vox): while they play aac (.m4a) apparently correctly, they misunderstand tunes'durations (displaying something like 1:13:24 instead of 04:35).
It might be boring if you have to go to a precise location in the song, when checking a converted song, for example.
The only app displaying durations correctly (except for iTunes or Quicktime Player, of course) is VLC!
P.S.: note than I have only checked on PPC :-)