XBox Music is free for Windows 8 users.
You have unlimited music streaming (with ads) for the first 6 month, then you have 10h per month of listening.
If you get the subscription which 10$ per month (or 100$ per year, or cheaper if you watch for special during Christmas time. I got mine at 50$), then you have unlimited music streaming with no ads anywhere, and unlimited music download (playable on Windows 8 systems, Windows Phone 8, and as well with iOS and Android (with XBox Music app). In addition, as part of your subscription, you also have access to the Zune marketplace for the Zune desktop app (identical music store, unlimited streaming and download). This is offered for music streaming/download on Windows 7, Vista and XP systems, and so that you can transfer your downloaded music to the Zune MP3 player series, and Windows Phone 7.
I have the XBox Music, and I think it's awesome. And an amazing deal if you listen to a lot of music.
The only thing I wish it had, is the ability to just play random songs of a genre. Like a Radio station by genre.