Another fairly new member here.
I have a Samsung Galaxy S (Vibrant) and was just playing with this last night.
After reading reviews on different music player apps for it I grabbed PowerAmp and started playing around.
Almost all of my music is in Apple Lossless format so I used that for my testing.
My normal portable rig is a 5th generation iPod, Pratical Devices XM5 and Ety ER4s. I plugged the phone into the XM5.
I'm sorry to say the phone is not in the same league with the old iPod.
The phone is not as clean sounding.
There are some up sides though, the EQ in the PowerAmp application is better than the old iPod EQ. With some additional software it looks like you can actually sync to iTunes. Of course you can also just put the audio files on your phone and there are several ways to do that.
I have to wonder if it would be better if you could get a line out on the phone. I hear that it is possible but I think you have to have some sort of Samsung dock for it.
So, after spending the evening messing around with it I decided to stick with my iPod. The phone just did not perform on the sound quality front.