The 6CD system sounds pretty good, certainly far better than any of the Forester or Impreza systems. While it doesn't have feel-it-in-your-chest bass and you won't be winning any sound pressure competitions, it's a reasonably good factory stereo.
I upgraded the unit in my 05 to an 07 unit. It has the SRS sound enhancement which while minces up the sonic reproduction, overall the sound is a fair bit better than the 05 unit. I did find with the SRS turned off, the unit sounded worse than the 05.
It's all very subjective. What passes as good for me might suck for you.