I have to say that I'm pleased with the Z as a mp3 player.. except, of course, it's ENORMOUS by comparison.
But, I don't listen to music at the gym, so it's fine.
I often use mine with just the built-in speaker ala transistor radio.
Or, I connect to a samba share and then have all my mp3s on my desktop/server available to the Z wirelessly.
When I do use headphones (
see this thread for 2.5 -> 3.5mm adapters) I find the clarity to be great and can pick out parts of the mix that I don't typically hear thru regular speakers.
I suppose this is all subjective, but no complaints here.... just don't take your Z on the treadmill and drop it (even though it should be able to survive the fall.. I've read nothing about it surviving being stepped on)