Has the game helped or hurt your real instrument playing? My poll lets you pick both because it has both helped and hurt me.
It has helped me learn the actual songs. Often the bass part in particular is mixed very low on the real song and is boosted in Rock Band. Case in point: ...And Justice for All. With No-Fail or Practice Mode, I can isolate rhythm from lead guitar as long as the game's version of the song was mixed so that they are separate. The note charts give me an advantage over just learning by ear by giving me hints of the rhythm. The game has also exposed me to genres of music that I would have never found on my own, and in particular has cured my fear of metal music. Before the game I thought any metal that wasn't "hair metal" was death metal. Hell, when I was much younger (6 or 7) I thought the Beatles were some kind of metal.
It has hurt me because the volume I play the game has taken away from time with the real instruments.
Drummers probably do not get to answer. Expert Drummers are already playing the real song, minus cymbal, hi-hat pedal, and second bass pedal. I am not a full Expert Drummer so I would not really know, though.