I notice it under 2 conditions: THe mp3 is a low bitrate or is encoded terribly. Anyways, when I download mp3s, I get them at 192 Kbps or the highest available if 192 isn't there. Then there's also a massive amount of mp3s I rip myself from my own CDs. So, I make sure they all sound good before I put them on my playlist or CD for my car. Also, yes, it does play regular CDs. Don't get Aiwa or Jensen decks, they suck. Kenwood could be consider a lesser brand than something like Pioneer and the like, but I've been pretty happy with my Deck. Also, don't get Sony, they cost too much, delievr too little and many people hav ebeen very unhappy with them. Which reminds me, I have a regular Pioneer CD Deck that came from my car. Anybody want to buy it from me? 
(Ill get the specifics if anyone feigns interest).
(Ill get the specifics if anyone feigns interest).