Bought a car stereo a little over 2 years ago, the stereo system could either connect to bluetooth, or use a USB device. Connected my phone to bluetooth, it didn't sound right. Couldn't put my finger on it, but it sounded thin. Put the same music on a USB device, it sounds just fine.
I've read, but cannot confirm, that the problem is that compressed music on your cell phone is decompressed and then recompressed and decompressed again over Bluetooth.
The problem is that the compression algorithms are designed to emphasize and de-emphasize different music characteristics. So it's kind-of like squeezing your music through two incompatible audio filters, it just squeezes all the goodness from your music.
In fact, I don't know but I'm willing to bet the Bluetooth compression is optimized for speech, which would be crap for music.