I have a zen x-fi and a couple ipods.
Let me state without reservation, the sound of the zen x-fi *destroys* ipods and the rest. It's not even close.
The x-fi crystal eq really enhances the treble, in most cases. Some instances it will over do it , but 90% of the time it's amazing.
The X-fi also enhances left/right channel separation in a way that no other mp3 player does.
I have listened on a variety of headphones ranging up to $1,100 a pair (ultimate ears) and the x-fi is a hands-down winner, the REAL end of story.