I don't think Genius is anything like Pandora. Pandora analyses components of the song such as instrumentation, form, meter, etc. while Genius only analyses the artist and genre.
For instance, the Genius playlist for a song from the score to Finding Nemo gave me a song from Hedwig and the Angry Inch. Sure, it's a soundtrack, but is there ANY relation other than that? Not at all. I could do the same thing by sorting by genre and putting it on shuffle.
I hope it gets better sometime, but there is no way to 'thumbs down' a song that you dont think fits in the playlist like you can on pandora. It will probably take a while to really understand people's listening habits.
EDIT: also, when I made a playlist of "Sell Out" by Reel Big Fish and then "Drunk Again" by Reel Big Fish, I got very very similar results. Sell Out is a ska song that's fast and has a prominent horn section, while Drunk Again is a Wurlitzer-driven 6/8 bluesy ballad. I think it might only be based on Artist right now...