I traded my old 335i for an M3 on Tuesday this week. The 335 had the Logic7 and the M3 has individual audio. I think the difference is quite big..
The Logic7 has more of a softer sound as the individual audio sounds a bit clearer. The real difference comes in when you start putting the volume up, you can really hear that the individual audio has more power. Eg. when listening to music with heavy bass, the upper parts of the sound remain as they should and are not "dampened" by the lower end.
So I would say that if you like to listen hard, the individual audio is worth it.