I have not used one, but I heard one in a truck recently, and it's almost completely smooth. Not as smooth as a stocker, but very civilized.
There are a couple of videos with sound of a Vette w/ZZ4 here: http://www.c3sharktank.com/sound/sound3.html
Listen to the top 3 from Sigforty. The truck I heard seemed a bit smoother, but it had quiet mufflers.