Just a qucik review:
I have (or I guess, had) a Zen, and completely loved it. The sound quality is incredible (and, yes, better than the iPod I have now). I really liked the navigation system and the ability to manipulate playlists on the device (which the iPod can't do). All in all, I was incredibly happy with it.
That is, until the hard drive died about two weeks after the warranty expired. Though it seems like the fact that the warranty had expired didn't make much difference: I sent nine e-mails to Creative's tech support department. Only one was ever answered, and that was answered with a form letter that had little to do with what I had written them about. When I was able to reach someone at one of their phone #'s, which wasn't often, I was repeatedly told that tech support for the zen was handled only by e-mail and they couldn't help me. I sent them a letter U.S. mail, but three months later, still no response.
I by no means suggest that if you buy a Zen, yours is going to stop working just because mine did. All products have a percentage that fail before their time. However, their customer service is clearly a joke, so IF you have problems, you're likely going to have a lot of trouble getting them fixed.
I was so mad after the Zen failed that I bought an iPod. (By that time I had become too hooked on having my mp3's with me to go without.) It's just as awesome as everyone says it is, but the price is difficult to swallow. And I can't in fairness say that the 10 gig iPod is worth nearly double the cost of the 20 gig Zen.
Good luck.