I've had one for a couple of weeks now and I like it. Sound quality is good given that it is playing compressed audio and it is pretty easy to use. The buttons are a little difficult if you can't see them as the feel is the same on both sides.
The unit is a decent size although bigger than an iPod but still managable. I tend to put in the inside of my coat pocket. Bundled headphones aren't great but they are functional and I've used them as backups.
If you are considering this then check out the following sites:
http://www.nomadness.net for good information about the unit. Also the bundled software is not great so I have exclusively used NotMad Explorer from Red Chair Software:
http://www.redchairsoftware.com Great piece of software.
My main decision for this over the iPod was price and what I perceived to be better PC compatibility and functionality. I travel alot and have limited space on my laptop so I use my nomad to store large files or archives.
Battery life is decent and if you turn it off and connect it to the USB port it will charge (slowly), I have left it on and connected to the USB port and the batteries will drain.
Let me know if you have other questions and I'll do my best to answer.