Unfortunately, most pro audio apps such as motu's Digital Performer and Cubaser VST 5.0 run only in 9 (at least for me, ... couldn't get them to run in OS X). Problem is, for midi setups, a lot of these rely on Opcode's Open Music System (OMS) to define the studio and instruments.
Also, my apps seem to run faster in 9 than in OS X, maybe due to the fact that I can allocate as much memory (I have a gig of ram) to the app, so it has more resources than with OS X's memory management. Who knows.
🙂