OGM is, fundamentally, just a hack of the Ogg container format, which was not designed to contain both audio and video. Because of this, it has some limitations that will hinder its acceptance as the next big container format. New container formats like Matroska will probably win over OGM in the long run.
At first, a major drawback of OGM was that it was not open source even though it was based on and around an open source Ogg framework. Apparently, Tobias was embarrassed about the quick-hack quality of his code and wanted to improve it before releasing it to the keen eyes of the Ogg Vorbis team.
However, Tobias has now officially joined the Xiph.org_Foundation team (Xiph created Ogg Vorbis) and donated all the code to his directshow filters, including OGM-format manupulation and playback, to their "BSD-style" opensource repository. It is available, though without documentation, in the "oggds" module in the Xiph source code repository.
OGM Media Files are used often with the XviD video compression, although once Ogg Theora is finished, it should be the best codec for the OGM format.