MP3 CD players look remarkably similar to your standard run-of-the-mill portable CD player. In fact, all of them play standard audio CD's, but their main feature is that they can play CD's with MP3's on them.
But what's the big deal?, you say. I can burn my MP3's to a CD that'll work on nearly any CD player. You're absolutely right, but let me elaborate.
When you burn a CD with MP3's that works in any player, those MP3 tracks get uncompressed into standard CD audio. While this allows them the liberty to play in just about anyting with a CD drive, they also take up a lot more space than the original MP3's. Those of you with a CD burner will probably already know that the average blank CD holds 80 minutes worth of music.
This is all fine and dandy, but did you ever realize that you can fit hundreds of MP3's on a data CD? Indeed, some of you have probably made these "MP3 cd's that don't play", either by accident, or becauce you wanted to transfer your whole collection onto another computer. And I mean whole collection. You an fit about 10 hours worth of standard-quality MP3 files on a CD. Most people couldn't even fill half a cd with their MP3 file collection.
Still though, wouldn't it be cool if there was a cd player that could read those MP3's off the cd, and then play them all? Yes, and that's exactly what an MP3 CD player does. It's the ultimate tool for those too lazy to try fitting all the songs they really want onto a mix CD. You simply burn your entire collection (or a good chunk if you've got an ungodly amount of MP3's) of MP3 files to a CD, pop it in the specialized player, and once turned on, it can read and play all of them. Even those of you who think that downloading MP3's is Communist or something can use this device. Take approximately 100 music CDs you own, rip mp3's from them, arrange the tracks in folders by album, and copy them all to one blank CD. Viola. Who needs to lug a huge CD case into the car when you've got everything on one disc that can stay in the player? Even better, any MP3 CD player worth it's salt can be configured to only play one folder, so if you want to listen to a specific album on your CD, you simply pick the folder and you're off.
Speaking of MP3 CD player's worth their salt, I shall provide a warning. Right now (as of 6/30/02), you're going to pay $100 minimum for a decent portable that doesn't skip or have piss-poor battery life. $200 is about where things top out, though, and prices are dropping all the time. Just do yourself a favor and read a few reviews on whatever you decide to buy, so you don't end up like my stepfather, impulse-buying what turned out to be an $80 paperweight.
Oh, and there are disadvantages compared to other portable MP3 players. You can end up paying more for a memory-based MP3 player that only holds an hour or two of music, but they have the advantage of being skip-proof. If you're a jogger, you may want to skip on an MP3 CD player, as the current technology just isn't sophisitcated enough to ensure skip-free playback of your CD while you're bounding down the block for a daily jog. Hard drive-based MP3 players can be much more expensive, and they aren't skip-proof either, but they're also usually smaller, and can sometimes hold even more than an entire CD (Apple's newest iPod has a 10 gigabyte hard drive, for example).
Overall, though, there are plenty of good reasons to buy an MP3 CD player. Just make sure you know what you're getting before you go out and plunk down serious money on one, and of course, ensure you have the capability to burn the MP3 CD's.