My experience is with the Clip+, but everything indicates the Clip+ and Clip Zip are equivalent in SQ.
The Clip+ is better than my Cowon D2 and my Android phone, and with my Grado SR225i headphones, just as enjoyable in SQ as listening with my much more expensive Asus Xonar Essence STX sound card on my PC (some of my other full size headphones are better on the STX). The Clip+ will also drive very low impedance earbuds/IEMs very well, something my STX will not.
My 12 year old son lost his Sony E Series Walkman this summer. Replaced it with a Clip+, and he immediately noted the improvement in audio quality. I had listened to his Sony, and I agree.
Everythingbutipod reviewed the Clip+ and found it to be comparable in SQ to the Cowon series of mp3 players:
In this very technical spec review,NwAvGuy found the Clip+ to be fairly comparable to the iPod touch, even exceeding it on some performance metrics: http://nwavguy.blogspot.com/2011/02/sansa-clip-measured.html
If you want to see lots of user debates over the SQ of the Clip vs other players, read this thread at Head-Fi
.Edited by cel4145 - 10/17/12 at 7:38am