I've had the car for about 3 months now with an upgraded version of the RCD-510. Wife has the previous version of the RCD-510 in the Tiguan.
I've never really fiddled with hers but now that I'm using the JSW as my commuter instead of the Audi, I was getting really sick of what I thought was the crappy sound of the factory stereo. So much that I was looking into new speakers, etc. (remember back when MBQuart had a complete upgrade set for factory stereos?).
I'm not a stereo guy. I just want decent sound coming out. Clear, no crackles or hiss and that's what I was getting with my iPod connection. Until I took the time to upload my music to an SD card..That's when I was impressed! I can't believe how much better everything sounds now. Soooo much clearer and so much "presence" to the music.
Like I said, I'm not a stereo guy. For the high-end people, you probably already realize this but for everyone else, give it a try. I'm sure you'll be impressed.