I've fitted an Aux in to my NCDC 2013 in the 3.2, I personally find the audio fine.
My Omega is 15 years old and has wind noise and engine noise to match, so don't personally find any impedance issues.
I use an iPhone with Spotify linked up, music turned up sounds far, far better than radio and make full use of Bose and bass. Huge improvement.
As Zirk says, the iPhone is particularly shite at directly pumping in music. The tones aren't massively out (to my tone deaf ears), still noticeable, but the volume levels are complete wank.
You need the iPhone at absolute full volume, and the stereo cranked up a bit to make it audible. And then shit yourself if you have a travel announcement or satnav come through.
Not a problem, as I use phone for sat nav and traffic.
I nearly lost an iPhone X in the states, because of its fickin' hopeless nav. In 2 weeks, we must done around 150 u-turns.
The phone's saving grace was it wasn't mine.