I put together a music mix on Panodora. Something to have on the headphones that frees up my mind but doesn't take over my head space. I also listen to Chill Out 2 New York through Live 365.
Neither are perfect. I was shooting for a down tempo-ish mix on Pandora, but Pandora can be random.
My preference is the music selection that is streamed out of Chill Out 2 New York, but I really don't like the interface. I don't mind the ads - it's the user interface that mucks up my mental map of what is going on with what I have open on my computer. I need mental clarity when I work. Live365 who hosts the station, attacks this. They make me spawn a player window and then spawn iTunes (the player I prefer from there) to listen to the music. In so doing you lose touch with the songs - information about them and where to buy.
Pandora's winning play is to have the music player on their home page so there is not a lot of crap spawning all over my monitor. I also like the connection to the music more. There are screen shots of the album covers, the ability to get background info on the music and seeing what others also listen to. Pandora is also a different beast than Live365 - it builds it's music list from music that you've seeded; providing selections based on what it imagines is your preference.
This leads to finding new music but also to some disasters. For an example, I don't want female vocals as a rule in my chill out mix, but I like Sneaker Pimps' 6 Underground. I chose that once for my Pandora Designer station and got an endless stream of music from Madonna and onward which didn't fit the vibe I wanted at all. Pandora does not deal with balancing music choices well. It's not an option. So the one-offs that you like can throw the thing into a tail spin.
You'll need to register for Pandora and Chill Out 2 New York (at Live 365) but don't worry too much about that. Friends and I have been using these services for a while and nothing horrific has happened; no spam or weirdness.