I just spent a few minutes playing with PortMIDI in csound 5 on Windows. It 
seems to work just fine now. I didn't do any actual timing tests;but 
subjectively, latency, consistency, and stability are excellent. I was able 
to stop and start performances numerous times. The midiinterop stuff works 
too, so I was able to test with several "regular" Csound instruments.

I used an M-Audio Ozone USB keyboard that does both MIDI input and audio 
input/output.

Thanks a lot, John!

I will be building with PortMIDI enabled on Windows from now on. What is the 
story on Linux and OS X?

I have been building the Linux snaps with PortMIDI for a while, and
will continue that way.  I think some one more MIDI based than I
should kick it more though.
  I will look at the OSX version in the next day or so.  PortMIDI is
supposed to run on OSX so I am not expecting problems.  There is no
OS9 implementation, but I suspect that is less important.
  I also would like to look through the later parts of the MIDI
operations, worry about SYSEX stuff, and so forth.
  Mind you, I am greatly relieved that it seems to work!
==John ffitch