Albert Graef wrote:
  Hmm, 20 msecs at 48KHz doesn't sound too bad. The
earliest pitch
 trackers on MIDI guitars had some 250 msecs latency IIRC, now those are
 a real challenge to play. ;-) (Robert Fripp did it, though.) 
In a music store I played Voodoo Chile on a guitar synth in the middle
80ies, the synth sound was played with a latency that might be something
near to 250 msecs for the lower strings, the high strings had better
latency. De facto playing was impossible.
As far as I remember Buckedhead played a well working guitar synth for a
project with Bill Laswell and Bernie Worrell on one of the Praxis
recordings in the middle 90ies: 
http://en.wikipedia.org/wiki/Praxis_(band)