ive an ipod and my roommate has a 20 gig zen.
it seems to be pretty nice, easy access of songs. hes kinda messy though so theres not any playlists made. and not many of his songs are labeled correctly. but... it is fairly easy to search through songs and albums. it does auto create them (playlists, that is) from the id3 tags. the jog dial doesnt seem to accelerate so it can be a pain going from the top of a list to the bottom.
the creative EAX dsp modes are fun to listen to (hall, theatre, bathroom, live, opera and others) but the equalizer leaves alot for the wanting. its got your standard presets and a custom mode. only like six or seven (iirc) bars and no way further customization than your standard up and down. (ive now compared it to empeg. of course it wont satisfy me equalizer wise) it can serve as a portable hdd, which is nice, but cant everything these days?
it supports recording through a wired mic, not sure if its included or not, never used it.
umm... not much else off the top of my head.
over all... ipod is smaller and weighs less, is much prettier and has a coo jog dial on the front.
nomad is little heavier, kinda confusing interface at first, has cool sound distortions and i think is cheaper.

both, imo, are overrated. but id pick the ipod over the zen.