Playing with it now and it seems pretty good. The trending chart is obviously full of the usual suspects (Psy is #1 right now). The emerging chart is actually quite good. I haven't heard of anyone on it but already heard a few good songs. The suggested chart (based on artists on you follow which the app finds automatically when you link your Twitter account) is also quite good. It's finding me tracks by artists I know but haven't heard in a while and a few new ones too.

The play interface is a bit clunky. I've already accidentally switched to the next track and there doesn't seem to be a way to seek so when I went back I had to start the previous track from the beginning again.

The app makes sharing what you're listening to pretty easy and includes a link to the track on the music service you've linked your account with (Spotify or Rdio).

Search is a bit funny. It says 'search for artists' but really just returns a list of twitter users matching your search term. I could see twitter introducing some sort of artist account. It can't be that hard to verify an artists authenticity through checking if they have music on Rdio, Spotify, or iTunes.

It seems like it could be a good service and is soemthing that will be useful to me immediately but they do have a few things (mainly the playback interface and search) to work out.

Edit: The website is now live: https://music.twitter.com