Just to be fair, there are many different setups that your friend could be using. Directv has XM compressed one way, the internet compresses it differently, and finally, the regular receivers do it another way. If your friend is playing back XM via the internet on a main system and is not using their top internet quality, it sounds pretty awful.
My friend has the Polk XM system and he has had it on in the background, but I have never experienced a headache. YMMV