These discussions are always fun. I personally have played Guitar Hero and did not enjoy it. But, the reason I did not enjoy it is because I am a guitarist, and playing Guitar Hero is a totally different thing than playing a real guitar.
First, let me say up front that I initially had the same reaction as many of you, being a musician, and I am not trying to put anyone out for their comments, but I think we should look at it in context..
We must first realize that we are not the target audience here, anymore than we are the target audience for MarioKart or any other video game. Sure, we have a Wii, we play it, and I a lot of my peers play too, but we are not the main consumers.
Second, we must consider the reason for this game. It is a game - it is not intended to be an instructional tutorial on how to play an instrument. There are people that play RockBand/GuitarHero that have absolutely no desire to learn a real instrument. They are just playing a game. In that respect, it is no different than Madden Football - should we ridicule everyone who plays Madden Football as pretenders, and question why they don't go out and play real football? Do real football players make fun of people who play football video games?
Having played Guitar Hero, although I sucked really bad, I can completely see how this game can help a kid learn rhythms. I do not believe that GH or RB can help anyone learn anything about the guitar or singing, but it can absolutely help with learning rhythms. The hardest part of GH is that you are not really playing the guitar part - you are hitting different buttons on the controller at appropriate times with the rhythm of the music, so there is no doubt it can be a good starting point for learning rhythm and staying in time with music.
I look at it this way - music lessons are boring, and suck. I took 2 guitar lessons when I was 12 and quit because it was so boring. Instead, I listened to records, bought Tabs and learned the stuff I wanted to play, and became a pretty good guitarist. That said, I wasn't really really tight rhythmically with a band until I actually started playing in a band in high school. Even though I was good playing by myself, it took jamming with a real band to get that rhythmic sense down. RB might be an alternative way for kids that can't form a real band for whatever reason to get some of that rhythmic sense down in a different way.
Heck, my 5 year old son plays the crap out of the hardwood floor with a pair of chop sticks....maybe I should tell him to stop pretending and get a real drum set

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