Jeff,
Ever since the second (they are up to the fourth) interim release it has behaved pretty good for me.
UAD and Plugin Alliance plugs were having problems but on the third interim they got that sorted out (for most, there is one guy who says he crashes with UAD on a MAC).
I think some multi-out synths are still not working (EZDrummer 2 comes to mind). But I'm not that full tilt interested in the MIDI. I don't really plan to mix/master projects in MB3 using any MIDI at this point.
I am running a test project with MIDI using IVORY and about 20 audio tracks and it works as expected.