Well, I have the iRiver ChromeX, which is SlimX's younger brother, and I simply love it.
Really cheap, really good features, slightly thicker, but you can use normal batteries inside it, unlike with the SlimX. The LCD is on the unit, though

But the rechargeable batteries that the SlimX uses are probably quite good...
The RioVolts are just iRivers rebranded, so... I'd rather go for the originals

(Nevertheless, I also bought not long ago a RioVolt Sp-90 for my girlfriend, flashed it to the iRiver iMP-100 with a small hex-editing, and it works wonderfully well...)
I'd say iRiver really knows their stuff. You just can't go wrong with their products. And I think the whole family will get Ogg playback, not just the newer models... (They seem to develop firmware updates in parallel for all of them... a common API, probably)