I've been using Pandora occassionally. I find it can get repetitive with songs, not within a single listening session, but if you close that session and then go back to the same channel later. It makes it feel like the song selection is limited. Overall I've enjoyed it, it would be nice if they had a record of past listening history instead of just the current sessions to better be able to give out new music. Obviously, that would be a lot more to keep track of on their end, which is likely why it isn't done currently.
