The music sounds excellent with the MiniJam.  I listened to the included songs and a few of my own, and songs like "Bohemian Rhapsody" had no distortion during its a cappella portions.  It is as if I had my own portable concert!

All of the MiniJam's hardware buttons have a dual function if you hold down a button for a few seconds.  You can customize them by going into MiniJukeBox then Playback | Preferences then tapping the Buttons icon.  Here are the buttons and their defaults:
 

Button	Press once	Press and hold
Play/Pause	Press once to play & again to pause	Stop
Next/FFW	Skip to next MP3	FFW within an MP3
Prev./Rewind	Skip to previous MP3	Rewind within an MP3
Volume Up	Increase Volume	Continuously increases volume
Volume Down	Decrease Volume	Continuously decreases volume
MJ Control (circle)	Run MiniJukeBox	Puts MiniJam in sleep mode
Sound Control (square)	Cycles Sound settings	Mute

Playing songs is just like a normal CD player.  One thing that I noticed is that the skins don't have a pause feature; I spoke to InnoGear about this and they will be adding this to their skin creator.  Tapping the Play button starts the selected song from the beginning, and tapping it again stops the song and resets the timer to 0:00.  You can, however, use the hardware Play button to pause the song.

When playing a song, you can switch to another application and the MiniJam will keep playing. You can then use the MiniJam's hardware buttons to change volume and track selection. Press the MJ Control (dot) to return to MiniJukeBox. There is no slowdown at all in any applications that I could see; in fact, PocketMark was 3 points higher when using the MiniJam!

FileMover

Ok here's the bad news: you can't store non-MP3 files on the MiniJam's MMC -- yet.  InnoGear, after taking a lot of heat about the MiniJam's delays, chose to release the MiniJam first and add the MMC file management feature later as a software update to the MiniJam.  While I'd like to see this feature now, I'm glad to be able to use the MiniJam for its intended purpose now, not later.  I will say that ROM updates are possible on the MiniJam, as I did one over the course of my review.  This review will be updated when the data storage feature becomes available.