I just treated myself to a 60GB Zen Xtra, and I couldn't be happier with it. I researched ALL the different kinds of mp3 players out there for hours, and basically they all have their pros and cons. Quickly, the pros for the Zen..
-replaceable battery (some others (Ipod) don't)
-long lasting battery life
-crisp sound
-tons of space (16,000 songs)...don't think you'll need all that space? just wait...mine is half full already
-best bang for your buck (or pound)
-Normalization...I have mp3's from everywhere, this will normalize the volume on all of them, Ipod doesn't have this feature.
Cons
-bigger than Ipod
-can't run with it
-supposedly clumsy software, but I've had absolutely no problem with it.
I picked mine up for $279 USD and couldn't be happier, I love it. You're adding about 100$ for the Ipod name, truly. But it's alright because I own their stock.
good luck