Well, I'm new to the forums here, but since I have a Rio Volt, I decided to post my $0.02 here:
Basically, the unit looks very decent. It feels weel made, not like that MpTrip stuff, that looks like its held together with glue (I have one...I should know).
Anyway, it's not overly big either...since it's round, it makes it smaller. The buttons and d-pad is all very functional, which I like. The only gripe I have about the buttons is that there is no way, as far as I can tell, to skip like 17 tracks for example. You have to skip 20 using the +10, and go back 3, etc.
The illumination on the screen is a very nice blue-green, and it is very legible. It displays track time, track #, filename, battery power, and any modes like shuffle, etc. The letters are nice and big, which is bad in a way because it takes a while for the name to scroll across. Also, you can navigate directories, which is very very cool. Just an observation...once you load up a cd, it reads the FILENAME. Once the track is played, it loads the ID3 from it. So unless the song has played already, browsing through directories can be a bitch because if all the filenames start with ARTIST - TITLE, all you will see is artist initially until it scrolls, which takes like 2 sec. However, if the ID3 tag is read, it will display TITLE - ARTIST, which is what you want if you have multiple tracks from the same artist.
Sound quality is great. The line-out is good too. It may/will have trouble with generic ac adaptors/cigarette adaptors, as my MpTrip did. Batteries are 100% though.
The remote control is very functional too. Its a tad sensitive, so you might turn it on accidentally, thus wasting batteries. Also, theres no +10 skip button on there and no way to skip multiple tracks at once using the remote, which blows...yet they have a useless button for EQ on there. Oh well. Also a bit difficult to press the side buttons too.
Haven't checked out the other stuff it came with. The case, the software, etc. You probably won't use them much though.
The caching is cool too. It reads for 10 sec or so every 80 sec, so its semi-solid state I guess. Haven't made it skip yet.
If you can, buy this unit. It's very good, and firmware upgradeable. Like all things, they will get better in time, but at least this can be updated to keep up with new codecs, etc. I got it for 180, which is worth it in my opinion. Its probably the best unit out there right now and will be for some time I think.