The gang at Extereme MHz have done a brief review of the 20GB Rio Karma.
There is nothing to dislike about the Rio Karma. It offers you much more out of the box than the Apple IPOD does and is the absolute best alternative currently on the market. It's the most feature-rich hard drive MP3 player we have had the pleasure of reviewing. Its superb playback features overwhelmingly surpass those of the Creative Nomad Jukebox Zen. The player, as well as the docking station, are superbly designed and allow you to not only connect it to an audio component, but also offers some advanced networking capabilities not found on any other digital audio player. Its design, display and interface is top notch. Based on my research, the only thing in question is the overall build quality of the device itself. I've read quite a few horror stories regarding this player. From hard drive failures to broken riosticks and wheels. I cannot comment on these as I have not used it long enough. Sure, all devices with moving parts are capable of malfunctioning, but if purchased with some kind of service plan or warranty, you'll at least have some peace of mind. Most users, including myself, strongly recommend you opt for a service plan when buying these types of devices.
That's some pretty high praise, but one big negative in our book against the Karma is that it is not a USB Mass Storage Device (MSD). This means you can't plug it into your computer and dump files on it. First you have to download and install the Rio software and use it to transfer files to the device. An unecessary PITA that iRiver and Apple models don't suffer from.