After years of effort, I found a station I enjoy on Pandora. That sounds dramatic, but consider my experience: I've registered twice in a period of two years and, disappointed, returned a couple times per registration optimistic that the library and algorithms would be improved. Basically, I found the results of my searches predictable, boring, or too unrelated to my expectations.
I was part of the problem, I realize. Why you're searching is more important than what you're searching for. I would turn to Pandora in those rare moments of iTunes-playlist boredom, search something like "Radiohead," then hope for flawless music. I'd leave dissappointed.
Eventually, Evan showed me a station he made for King Crimson. It came up with just what he wanted: experimental, progressive music from the 70's and 80's. I signed up again and searched The Album Leaf, wanting to hear more soft, instrumental, electronic-ish music. It didn't have to be "flawless" soft, instrumental, electronic-ish because I didn't really know what that would be.
Of the first 30 songs I listened to, I gave eight the "thumbs up," three the "thumbs down," and found the remaining 19 pleasant. I also downloaded four albums from two of 15-or-so artists that appeared on the station.
I'll probably never be a Pandora devotee (to much music library), but it's finally a tool I can recommend.