Once you've mastered easy with one finger you should skip medium, because it'll teach you bad habits, get you use locking your hand in one position. This will make it more difficult to adjust to using all 5 fret buttons. I of course learned this after I mastered it on medium, which is why I thought hard was impossible and I figured there was some ‘trick/cheat’ to playing it on expert.
After I re-mastered easy with one finger expert became playable. I haven’t mastered hard yet, but I am bored with medium and easy.
Yesterday Rockband released a patch that makes the crazier songs playable and stops the game for constantly putting in Metallica songs whenever you play a random song set. This should revive the game for me, since my friends and I thought we maxed out with our combined abilities.