I exclusively use Pandora. I've used it to a point where it has learned exactly what music I like. I've set up stations for different genres and even for different artists. Every once in a while I give a song a thumbs up or thumbs down and it does the rest. I haven't listened to MP3's on my computer in ages. Oh yea, and it's free. Give it a try. I've gotten many people hooked on it. It's also a once of the most downloaded Iphone applications.
http://www.pandora.com