I'm finding that BIAB (I'm running the 2012 audiophile version) runs without a hitch in Mountain Lion. However, if you are planning on working on files created in BIAB in Apple's own software, you might want to wait before upgrading. I'm finding that running Logic Pro under Mountain Lion is a nightmare, with the program suddenly becoming unresponsive to any keystroke or mouseclick -- there's a long thread in Apple's support area with other users reporting the same thing, the tracks just keep playing. There are always some problems, for some users, with any update, but this seems pretty extreme. Some users have also reported problems with garageband under Mountain Lion. The upshot is that my customary work flow, exporting BIAB aiff and midi files into Logic, no longer works. It appears that PG may have done a better job of testing its music software under Mountain Lion than Apple did.