I've had my Nomad Jukebox for some time now (it was 6GB but I upgraded it to 20GB some months back). Today I picked up iPod in the Elite store in Cupertino. I've been playing with the iPod today and there is a big difference between it and the Nomad.
Here is what I find interesting when comparing the iPod and the Nomad.
Portability:
Needless to say the iPod wins big time. The Nomad is HUGE compared to the iPod. I have taken the Nomad with me on planes and it is always pain, it is both bulky and I've had problems with the batteries. I usually take my RIO 500 (128MB) instead (not that it matters here, but usually I'm so bored of the songs I have on my RIO after a trip that I don't want to hear them again for months

)
Disk space:
When I moved from Europe to US I ripped all my CDs and put it on my computer. It added up to 35GB. When I was copying music on my Nomad (20GB) I didn't really have to select (many of my CDs didn't age that well haha). Today when putting things on the iPod I found myself selecting. If your music collection is huge the autosync feature is not very useful and 5GB is simply not enough.
Upload:
Here again the iPod wins big time (as everybody knows). It took forever to copy music to my Nomad (hours and hours) compared to minutes for the iPod. It may sound irrelevant because this is something one does rarely but the fact is that I never upload new stuff to my Nomad, because it is not easy enough. I rather listen to the CDs in my stereo.
Sound quality:
Pretty much the same (with good headphones).
Headphones:
Here we have two losers. When I got my Nomad I was shocked because it sounded horrible - but it turned out to be due to really bad headphones it shipped with. These are the worst headphones I've ever got with any device I've bought.
The iPod comes with earbuds. They sound good but they are too BIG. I've used a lot of earbuds and they have all been comfortable but the Apple ones hurts after 15 min use. I'll be using my Sony earbuds.
Controls:
I like most things about my Nomad but I really hate the speed of the interface. First it takes forever to "Boot up" (Prepare library). I can live with that, but flipping through list of albums or songs or skipping to the next song is painfully slow on the Nomad (if you haven't seen it, you will not be able to imagine how slow it can be - changing the volume just after start playing a song can take 5 sec from the time you roll the volume button until it starts changing the volume). The iPod on the other hand is super snappy. Much faster than I expected (again, if you haven't seen it you would not believe how snappy it is). It takes a little time from pressing Play until the song starts (same on the Nomad) but everything else is super fast. This is not as insignificant as it may sound. The usability increases with the speed dramatically.
Batteries:
I have only had the iPod for one day so I cannot comment much on the battery life but everything I've read is very promising.
I've had a lot of problems with my Nomad when traveling (both before and after upgrading the HD). Sometimes it doesn't even start, or stops when it has more than 50% left of batteries. I have two sets of rechargeable batteries. I followed the instructions for first time use, and have followed discussion about this problem. Bottom line, I can not depend on the Nomad with batteries. My friend has the same problem with his Nomad.
Overall:
I use my Nomad at work. Since it is always on my desk there is no need for batteries. Besides the slow interface and long boot up time I like it and I am going to continue to use it since it has 4x the disk space the iPod has. But I don't think I will upload any new CDs on it (it just doesn't happen) and it will stay on my desk.
The iPod I absolutely fell in love with when I saw it and my love just grew stronger with every minute I used it. I will always carry it with me and I will always have my latest albums on it. Before I bought it I thought it was a little expensive but now I'm fine with it. It was everything I wanted and much more (how often does that happen?).
Knowing what I know now, I am very happy having both. If I wanted a player ONLY for my home or office I would get the Nomad (4x more space - it matters). Another option, better than Nomad is to use your laptop or desktop where portability doesn't matter. But if you want to carry the music with you the iPod is the best choice you have. It also beats the flashcard players (64-256MB) big time. You can listen to what you want when you want. It is twice as expensive than most of them but in my opinion it is much more than twice as useful.
Thats my 2 cents