Not much to add beyond what's already been said. I never thought an audio panel would make much difference in the cockpit. After all, the audio quality is a function of headsets, or so I thought. Then on a whim and a Black Friday sale at @Avionics Source, I bought a PMA450a and later traded it for a PMA450b. It is the single biggest improvement in my cockpit.
Not only is it a top quality unit, but it's nice to know I can ask for service and get it, if needed. Garmin makes good stuff, but they won't give you same level of support at @jcovington describes above. They'll tell you to take it back to your installer. For those of us who are pretty good with our hands and somewhat technically savvy, being able to talk with the guys who build the kit, is a game changer. I had a GMA340 prior and did the swap myself. It took longer to open the box than it did to switch out the units.
Just some of the things I like:
Noise suppression is amazing. In my all electric airplane and two alternators, the PMA450 just took all the electronic noise away.
Music distribution and selective muting makes it my wife's favorite cockpit tech. Easier to get funding that way as well.
Bluetooth works and works perfectly. I took a call on my phone once while on the take off roll, full power. The guy I was talking to thought I was sitting in my office at home.
IntelliAudio - I'm always either on flight following or IFR. There's no need to ask to step away to get weather, or flight service. Its easy to listen to two channels at the same time.
@Avionics Source just made it too easy to buy, and @Mscheuer made it too easy to install and use. What's not to like?