I have been through them all, had an original 40GB iPod back in the day, iPhone when they first came out and now have an Android based phone from HTC (great phone, not so great for organizing music), but for music in the car out of all this stuff I found the simplest and quickest way to listen to the music I want - was to just copy it to a USB key and plug it in to the USB port in the car.
Before I had a car with USB support I just burnt what I wanted to hear to a CD as mp3s, at less than 5c per CD was worth tossing than having to go through syncing and hooking up your player to the car with cables every where.
Its the only way you can just grab what you want when you want and go, instead of fiddling with synching this or that and waiting for the various libraries to pat each other on the back, just a PAIN.
So while its great Google is coming out with a competitor to iTunes, I just prefer to keep it simple :)
Third party solutions tend to be a lot nicer. I’ve got an iPod Shuffle and with Winamp, managing it works pretty much exactly like managing a normal playlist. No synching, the only waiting involved is the time it actually takes to copy files.