I got a MiniDisc and I really like it.
If you get a Sony NetMD one you can load mp3 into it but you should know that it converts the mp3 to attrac3 which, I figure, is just their own mp3 type of coding and it's closed source so it can only be ready by Sony MD.
It's important to know that you can only load mp3s from your computer to your MD and you can only do it 3 times before you can't do it anymore. I would imagine there are ways around this but I have never been inconvenienced by this. The big thing is that you can only move back to your CPU what came from your cpu in the first place... you see Sony is big against piracy.
When it comes to recording your sets, the MDs are great but you have to record it off your MD to your cpu if you want to burn so if you have an mp3 that can records into mp3 directly, that might be better. But MDs are great for recording your own stuff, imho, because you can record yourself practicing and then you have it... so for me, it's like this: record at night and then listen to it the next day on my way to work, get home and just record over it if I want so it's a convenient learning tool. Because you can only transfer to your cpu using the software what came from it, you can't use the USB cable and just drag and drop to your HD.
I think that extra MD are like 5 a piece.. you can get 10 backs for just over 50 at Future Shop and a 10 pack should do you really well... you'll have a few for your mp3s and a few for sets and I haven't had to buy more discs.
MDs are good if you have friends that have them because then you can toss MDs around and trade them and stuff. But as I say about moving stuff from your MD to cpu you can't transfer what they give you back to your CPU.