I liked Pandora for about a week until I realized it was playing the same 15 songs over and over. I didn't do much customization of the channel, but I thought that would lead to more variety instead of less.
Also, and I have no idea if this is Pandora's fault or my screwup, it apparently sent some kind of invitation to several people in my contacts list to listen to my new Pandora station. I only realized it when two of the messages bounced back to my inbox. I definitely did not send anyone an invitation by choice, but I don't know if I hit something by accident.
I set up similar stations on Slacker and found it to have much more variety than Pandora. I've been listening to a single station for several days and I don't think I've heard a repeat yet. One feature from Pandora that I wish Slacker had was a way to see what's been previously played. Pandora had a nice Cover Flow view of the previously played songs that was very helpful.