MiniDisc players cannot play mp3 files per say. MD players instead use ATRAC which is a propritory compression system (codec). So your music files need to be converted from the original to ATRAC compression.
Now, this player comes with a program called NetMD, which will convert these audio files (like waves and mp3s) to that format. Although this software claims to tranfer at 32x, the speed depends on the ATRAC recording mode (4x is standard). It uses 1x USB Port on your PC.
Also important to note is the longer playing modes (called MDLP modes) like the (over 5 hour time you mentioned) is a very high ATRAC (ATRAC3) compression rate which suffers a great deal of dynamic loss and artifacts. Its much better for the spoken word like books on tape. Hard to say if your son will care that much...but its important to note anyway. This ATRAC compression rate can be adjusted to his taste.
The main disadvantage of MD is...once its on the MD disc, it cannot be transferred out digitally like many other digital devices. It's essentially "stuck" on the disc and can only be output in analogue and in real time.
Nevertheless, MDs are fun portable music devices. And since the blank discs are cheap (and easy to swap out), you will not be hurting for "memory space" as the mp3 players do (the main problem with mp3 players).
Have fun!
