I wrote this rant about ID3 tags (ID3 tags store the metadata about song name, artist name, genre, etc. in MP3 audio files) on someone's blog but I thought it might be helpful to let people know what they're getting into if they're unfortunate enough to be working on a project that requires ID3 tags and their contents to be sane and consistent and well organized.
I have a Rio 500 with 128MB of RAM. It is extremely cool. I have two small complaints: it doesn't have an AC adapter port (gotta use batteries all the time), and the switch on top which lets you disable all of the buttons (so you can put it in your pocket without accidentally fast forwarding etc.) is very easy to move too far, into the "off" position. Other than that it is very nice. A faster interface than USB would be ideal but it's not a huge deal.
I recently bought a USB to MIDI interface to control my drum machine / synth, but I haven't had a chance to hook them up yet.