I just started using the Slacker website to stream music and I remain very impressed at how good the service is. You are able to listen to a number of predefined stations or make your own. You can also customized existing stations by banning artists, etc.
The service is ad supported but you can choose to subscribe so that you don’t have to deal with the ads. If you do this, you also get to save some songs to your hard disk, get support for slower broadband connections and the ability resize your Slacker window.
In addition you can merge your own MP3s into custom playlists you create on the Slacker service.
There is also an associated portable music player which can synchronize with stations you create on your PC.
I have found myself listening to Sirius and Pandora in the past but now I have added this service to the mix. If you like Pandora (and I definitely do) this service is sort of like Pandora but seems to be more flexible in some ways. The user interface is not as nice as Pandora but what it lacks in simplicity it makes up for in flexibility.
Since Slacker is free, I suggest you give it a try