Let's see, I've had an original Rio, RaveMP, JaMP3, Nomad II, and a HipZip.
Of them all, I liked the Nomad II the best as far as all around qualitiy, price, & performance.
The HipZip was cool because the media was so cheap, but the player was expensive, so I was afraid to exercise with it for fear of dropping it (I bought it to exercise with).
The JaMP3 is tiny and cheap, so I'm not afraid to work out with it, but it is very cheaply made, and the memory configuration sucks (internal and add-on memory are treated as separate entities -- so you may not be able to actually use all the memory you have). Plus the software is horrible.
The Rio and RaveMP are so old, I won't even bother talking about them.
Now, I have a RioVolt, which I use ALL the time...and I use the JaMP3 for working out sometimes (usually, I don't use anything), even though its a POS.