Our car stereo is almost always playing CDs of MP3s I've made for the purpose. Like almost every CD player out there, the car stereo has a shuffle mode, and like almost every CD player out there, it sucks. Actually, I can't think of a single CD player I've ever seen that has a shuffle mode that works the way I'd want one to, so that it could be really useful.
The ideal shuffle mode would play every track on the CD once, and only once, in a random order, and then move on to the next CD. It would not use the same order on subsequent plays of that CD. And if you interrupted it in the middle of playing through a CD, such as by turning the car off, this wouldn't cause it to lose track of what has been played already and what hasn't; it would still do the same thing over many sessions of a few songs as it would over a single session of the whole disc. It would be okay for it to forget where it was if you took the disc out or changed out of shuffle mode, though.
It might be nice if it also allowed me to shuffle a whole disc, or just a subfolder of a disc, or shuffle across all the discs in the player.
This would allow me to organize the discs by artist, or genre, or whatever I wanted. My temptation is always to have the songs in some order on the disc, because you can always introduce randomness if you have order, but you can't introduce order if you have randomness. If the disc is sorted I can find a particular song or artist or album if I want to, but I can't if it's pre-shuffled. That's one reason the shuffle should work well: so I can use it.
But it doesn't remember what tracks it's played, so you can get one song ten times before you hear another one once. And it doesn't know when it's finished the disc, so once you shuffle a disc, all the other discs will never come up until you force it. I don't think it can even shuffle within a folder or across discs, though I'm not sure. So to get a reasonably good shuffle, I have to pre-shuffle -- burn the disc in a shuffled order, with all the tracks in one big folder, making it impossible to play it any way other than that one, single, shuffled order.
Clearly, the best solution would probably be a little MP3 player which I could load with playlists generated ahead of time. Pity that the Prius has its MP3 player auxiliary input jack located annoyingly inside the center console.