I joined and for $10 a month you get unlimited music - almost everything is on there. There are a few things I've search for that I haven't found, but only a few. It is so cool - especially if you have unlimited data like I do - I can listen all day if I like :)
You can create playlists, make them available online or offline, and come back to them later. You can just star a song and come back to all the starred songs later. Even if you don't want to join they have a Pandora-like radio you can use. If you do join you can use it on your computer, smart phone, and tablet (iPad).
I love it - revolutionizing how I listen to music. Sadly, the quality isn't great as with any MP3 service. The bitrate is too low, but that's a result of the streaming, at least with where the technology is right now.
Go check it out!