From iA wiki
From the same German development team as the original and hugely popular MP3, MP3Pro seeks to improve on itself based on competition from other smaller and higher-quality formats now available. Thankfully mp3PRO has no digital rights management features.
With a compression ratio of 20 to 1 that is difficult to discern from CD audio, one might think that MP3Pro will soon go mainstream. MP3Pro has not made significant gains, however, perhaps in light of its commercial interests and the existance of Ogg Vorbis, which has no or significantly fewer license issues. It has also experienced losses in the face of Windows Media Audio, which has found wide acceptance in portable devices and AAC, used by Apple's iTunes music service, and its portable music player iPod.