Rdio is the phoenix rising from Kazaa's ashes... and it's legal!
CD sales are at an all-time low. Disillusioned consumers are fleeing to the digital realm -- where music is cheap, where music is bountiful, where music is good. And there, awaiting their arrival, is a new breed of in-the-cloud music providers. Spotify is perhaps the service that exemplifies such a business model, but Rdio looks set to blow it out of the water -- to put it another way, I think Spotify knew what was coming when it dropped its prices last month.
In essence, Rdio is like iTunes and Spotify on steroids. There are no real surprises when it comes to the feature set. It's simply taking things to the next step. It's like iTunes, only without limitations on moving your files around. It's like Spotify, but cheaper, and with better music discovery. The coolest bit is that you can see -- presumably in real time -- what your friends are listening to. I guess it's also like Last.fm in that regard.
Now, both iTunes and Spotify are certainly working on similar features, so I doubt Rdio will be unique by the time it leaves beta testing -- but it's competition. $4.99 per month for unlimited music downloads from a library of 5 million tracks is pretty damn good... and it's only going to get cheaper!