MP3s are a thing of the past for me. It's all about the cloud now. I personally love Slacker, and haven't listened to any of my 10GB of personal music in over 6 months.
Unless if you're an audiophile, lol. I tried Slacker, and did like a lot of music it chose for me, but I NEEDED to have my music in FLAC and in MP3 CBR 320 for my MP3 player. I like Slacker and all, but I can't give up my FLAC and having a physical copy of the album. It's all personal preference choice though. =]