It's exciting seeing the potential in the "similar artist" radio feature, but Subsonic only makes a playlist based on the listed similar artists. If you have more than five similar artists, it will select five and leave the rest off. Reloading the page will bring up a new group of similar artists. I do not know how the similar artists data is pulled from Last.fm, but it'd be nice if we could create a similar artist radio using ALL of the similar artists you have, and not just the five it brings up.
EDIT: I am not sure how many artists it cuts off at, this artist page I was on had six similar artists. Maybe I can't count, but I believe this is still an issue.