I don't understand why people would bother with MiniDisc. I sell these things for a living, and myself and all of my colleagues (save the one who actually bought an MD player, just to be different I think) prefer mp3 players to MD.
For about the same price as an MD player, you can get a flash memory mp3 player that is a fraction of the size. You can get an expandable one, so you can use an SD memory card to give you greater capacity. The advantage to this is that those cards are coming way down in price, and they are more versatile than a MiniDisc. An SD card can be used in a digital camera or PDA, while MiniDiscs are useless outside of your MD player. At this level of mp3 player, you won't have as much storage space as an MD player, but an MD player still doesn't provide enough storage to hold most music collections, so regular loading/unloading of songs would still probably be necessary.
At the same size as an MD player, you can get one of the HDD-type mp3 players, ipod, ipod mini, Dell DJ, etc. You would have just as much bulk to carry around, but you would have more storage space. Depending on the size of your music collection, you could end up with all of your songs at your fingertips on a 4-5 GB mp3 player for about $230 to $250. The advantage is that you wouldn't have to regularly format and reload your songs when your mood changes (or bother loading in a different disc as is the case with and MD player) You wouldn't have to bother carrying around extra discs. Of course, this is going to cost you more, but it can save you money in the long run. A decent mp3 player will come with everything you need to use it and continue using it. There is no media to buy, and most all of the HDD mp3 players come with a built in Lithium-Ion rechargeable battery, so you don't have to worry about buying batteries or carrying extras. An MD player require you to by at least a small amount of media, plus batteries. MD players are very energy efficient, but it is something to be concerned with nonetheless. I think over time the extra money spent on a 4-5 GB mp3 player is money well spent.