The last.fm radio feature is very hit-and-miss, I agree. But it can be quite fascinating, if you look for tags and keywords rather than "xxx and similar artists". A while ago I sat around with some friends discussing time travelling (no bongs; don't judge), and we typed in "time" as keyword, and got some very interesting results. Since it's based on what other people are tagging the music with, we got everything from Spiritualized, Aphex Twin and 9 to 5.