Hi There,
I use my NHF800 (Hi-MD) primarily as a portable music player, and also as a storage device for my university stuffs. Yes you can transfer your downloaded mp3s to your Hi-MD set, or transfer the tracks from CDs you own.
The thing to know is - the "mp3s" you eventually listen to on the set are ATRACs, converted via Sonic Stage, the software that comes in your box.
If you listened to mostly live mix/dj mixes, you should know this too - if you 'burn' from CDs, the tracks are gapless and flow seamlessly. However, if you 'burn' from mp3s, there're discernible gaps between tracks, which might mar your enjoyment. (Using Foobar writing solves this problem sometimes [the results are not always perfect], but it means having the entire mix as a single lengthy mp3. But I digress...)
I used to own a Net MD before the Hi-MD. I'm happier now with the bigger capacity (naturally), the 6 band equaliser and surround sound options, plus not having to face the check-in check-out limitation...
So, I'm not sure if Sony will eventually go totally Hi-MDs. Even if they do, you should still get Net or other model MDs from other brands.
Rubi