I use iTunes for ripping my CDs to M4A files, and I use Mp3Tag to edit the tags.
However what I miss is the ability to view M4A tag data and cover art in Windows Explorer, and to use Windows Search to find tracks and albums. (Native Windows provides support for MP3 files but not for M4A). ... So I wrote my own Windows Shell Extension that adds support for M4A files to Windows Vista Explorer.
This shell extension can currently only reads the tags (no editing), so it is not competing with Mp3tag. But anyway it could be a nice extra for Mp3Tag users.