I have a Rio Karma (20 gig). So far I've been exceedingly happy with it. Very ergonomic design, FAST and logical menu system, very long battery life. It has great DJ mix settings, and keeps track of all kinds of user/song data. It comes with a dock, which has stereo outputs, and the unit can be connected via USB2.0 or conifgured to a LAN via Ethernet. It can also cross-fade between songs, which I really like.
Downside is that it has no FM tuner or recording capabilities, which don't matter at all to me, but you need to keep that in mind. I paid about $260 for it and it's money well-spent.
It generally gets reviews equal to that of the iRiver, though my roommate has an iRiver and prefers my Karma.
Also, the software for music transferring and organizing is excellent.